The Stonehenge Solution by Brian N. Johnson is a fanfic and it's free.
The Stonehenge Solution
Chapter 1 - After the war…
Harry Potter has barely gotten through the emotionally crushing weight of the last two months after the battle at Hogwarts and the killing of Tom. The unending days of Harry attending funerals and special events for family, friends, and people he never even knew weigh heavily on him. He misses his family and friends. They are not there when he turns to speak to them, having forgotten for a moment, that they are gone forever. And all the while, he is trying to be strong and supportive of everyone else. Let alone all those disappointed people who feel he should have done more and should do more even now. So he must ask, how much more must he carry?
Still, surviving all of the Death Eater attacks and the war has Harry believing the worst is over. But life is never easy, never forgiving, especially for him, The Chosen One. So when he learns that his two best friends have been killed in more senseless violence, it is the end. The last bit of weight that breaks his back.
After attending the funerals of Ron and Hermione, Harry was broken. Oh, he put on a convincing mask. Even at the inquiry, he was stoic and strong in the perception of the public. They all learned that a muggle, a mundane, a normal person, or whatever you want to label him, was responsible. He nearly reignited it all. But war and terror had taken all the fight from Harry and surprisingly most of the still living witches and wizards too.
One angry muggle father with a wife and two daughters gang-raped, tortured, and finally murdered. A father seeking vengeance and justice for his family, his world. Muggles don't have rights in the magical world he was told. His wife and children don't deserve justice he was told. A father's outrage and furry demanded otherwise though.
With desperation, this man sought an answer to the injustice. He sold many of his things, even the family home. A home cared for and improved by the family many times. It was passed to child after child for generations. With the large sum of money he now had available, he contacted friends of friends to purchase his tool of revenge and justice.
With twenty kilograms (44lbs) of high explosives in a backpack, he waited. He knew where to wait, he'd been there a few times before with his family. School shopping had come to signify something new and unexpected. Now hated. He could not see the pub's sign alone, so he waited for one of 'them' to emerge. It didn't take long to spot an oddly dressed woman just appearing from nowhere when watching the sidewalk closely. He quickly walked towards her and grabbed her hand so he could see what he needed to find. Dragging the surprised woman until he opened the door he found the entrance to his target. After letting her go he pulled the door shut. He walked determinedly through the old pub without saying a word. Then he reached it, the brick wall, the gateway to misery and pain. Someone called out to him. He turned with a smile as he pushed a hand into his pocket and flipped a simple switch. It was just a light switch he had stolen from his hotel room this morning. It still worked.
The Leaky Cauldron and a good portion of the Alley no longer existed. It was there one moment and gone the next. Dozens of people in the pub and behind the wall were torn apart just like the wall and buildings. Shards of glass, chunks of brick, jagged pieces of wood, and even slivers of bone, flew in every direction, furthering the range of death and injury.
Just emerging from the pub's fireplace was a tanned and happy family returning from Australia. Ron, Hermoine, and her parents never knew what happened. They like so many others were simply in the wrong place at the wrong time. Or perhaps they weren't, the father that just ceased to exist, thought everyone there deserved his family's justice. Was he wrong? Harry could not decide.
Afterwards, Harry retreated to Number 12, Grummond Place. Access was blocked for everyone after he and Kreacher locked the house down. Non-family house elves couldn't pop in. Owls couldn't find Harry and just flew back to their senders. Only loyal Kreacher went out to buy food and supplies for Harry and he always returned without messages unless they were from booksellers or Gringotts.
Harry's world now consisted of the house he inherited from Padfoot. A world he changed to fit his desires. All the windows were blackened so Harry couldn't see out, but spelled to still let the sunlight and moonlight in. Colors and decor were updated as well.
When Harry felt like not sitting anymore, he wondered about the house cleaning and repairing. Kreacher took cursed and other dark objects to the basement where Harry would destroy them with Fiendfire regardless of value. This included all of the dark items from the vaults as well. Dark books he kept, after curses were removed.
Most of Harry's time was spent reading the books from the library. A library that was constantly expanding. Every book and scroll from every accessible vault was brought to the house after Hermoine's entire collection was retrieved. Her copy of 'Hogwarts A History' was placed on a pedestal with a brass nameplate and glass cover. The nameplate listed her full name, birth date, and date of death. Below that it simply said, "Best Friend Forever."
Harry ignored Krestcher's behavior if he noticed at all when Kreatcher 'found' other books. A surprising number of Black and Potter books were returned to the library. One night the entire Malfoy library was added while Harry slept. A few walls were opened on occasion to make room larger when expansion charms reached their considerable limits with Harry performing them. Duplicate books were sold or traded for others. The funds raised were then used to buy more.
It was Kreacher who recognized that his master was happy or at least content when studying new books which drove the library's expansion. Kircher was devoted to fetching more for his master. He was rather uncaring about where they came from. Well no, the house elf was happiest when 'acquiring' lost or unattended books. This activity seemed to restore the elf's health and improve his disposition.
Eventually, two of the spare bedrooms were converted into additional library space. This is where the 'misfit' books were stored when Harry was unhappy with any.
Harry first studied mind magics until he mastered them. The initial effort to firmly grasp the subject took ten months, then he continued strengthening the skill with daily maintenance.
Next, he dove into runes and arithmancy until he felt he could pass the NEWTs for the subjects. All the while he continued making time for the 'core classes' from Hogwarts. Again he pushed until he thought he could pass the NEWT for each.
At some point, Kreacher asked Harry for permission to bring in another house elf for the House of Potter. Harry agreed and added Bob to the Potter family. Between the two elves, they soon were looking for magic books everywhere, and more places they shouldn't have.
This led Harry into enchantments, especially for the library. Once he felt ready, Harry and the house elves got to work remodeling. They cleaned out the attic and the house's top floor. Many of the items went to the second-hand shop while the rest went into the Black vault. Then they removed as many walls as possible to open up both floors of the new library.
The attic was converted and maximally expanded into a reading room with loads of transitioning skylights. With many bookshelves, a brass telescope, modern couches, a desk, chairs, plants, and more. The attic space was in perfect condition for reading and studying in comfort.
The top, or fifth, floor was given the same treatment as the attic plus expanded vertically for taller bookshelves.
Before adding any books to the new library Harry placed many enchantments on the entire house. Every square meter in the house would perfectly preserve books at the best humidity and temperature. Books would automatically be protected from water, fire, and insect damage. They would also be automatically cleaned, repaired, properly cataloged, and sorted onto the shelves with a Magical Decimal System, a new version of the Dewy Decimal System. A card catalog cabinet was set up for use. Each book added to the library will have multiple index cards for all the ways to find a specific book. His final piece was an automatically updating book listing every book in the library. The elves used a copy while hunting for more books.
Eventually, Harry's account manager and an American book dealer got a copy of Harry's extensive auto-updating book list too. That opened doors for selling or trading copies of Harry's rare books to others, usually for much older or one-of-a-kind books. Bob crossed the Atlantic so many times Harry asserted there was a path in the water between London and New York. Not to be outdone Harry's account manager got him first right of refusal on every book the goblins confiscated from vaults and access to many books in their library.
Black and Potter embossing stamps were linked to the library magic as well. Each book is added to the library upon getting a stamp (normally on the inside of the front cover or first page). Adding a book creates the automatically updating card catalog cards (especially location). Books placed anywhere in the house will always show the current location on the cards.
Automatic book sorting is done as each book is added. Books will float to a new location as needed when shuffling to new locations.
Additionally, books will be automatically ported back onto the correct shelf if taken out of the house by anyone, but Harry and his elves. Tapping a book with a wand from anywhere in the house will also port a book back to its spot on the shelves.
A never-ending logbook records every addition and update in the card catalog. And a chalkboard is magically linked for user searches through the card catalog.
After Harry finally got the library magic to function properly and just as he wanted he started enchanting a book stand. It has no legs, so it just had a flat surface to lay a book on and a lip to keep it from sliding off. Enchanted to stay floating wherever the user says to with, "bookstand hold" or to be moveable again with, "bookstand release" commands. Thus anywhere Harry happened to want to read a book it was hovering there and flipping pages automatically.
Harry studied like mad to learn Arithmancy, Astronomy, Blood Magic, Charms, Defence, Herbology, the History of Magic, Rituals, Runes, Potions, and Transfiguration.
Sixteen years after the war, plus the six years attending Hogwarts. That was how long it took Harry to get to the point where he felt he could pass an ICW NEWT test for each of the eleven subjects he had pursued so intently. That totaled twenty-two years focused on those specific branches of magic. Hermione would have surely done the same much faster, but he got there with pride.
There were still many more branches of magic Harry wanted to delve into, such as Alchemy, Healing, and the Mind Arts. They were certainly on the top of his list.
One subject entranced Harry far more than the others at this point. Rituals. So he continued his habit of reading and taking notes, mostly from sources from outside Britain. Most were from America or the East. Many branches of magic were not allowed in his home country, not that it slowed him down much.
For each subject, he had created a handwritten 'revision' book in memory of Hermione but they became much more. A truly large tome, that he constantly reviewed, updated, or added to. Each book is a complete guide to a dedicated subject, from beginner through OWLs and NEWTs. Eleven books in total. Each one of them has seven sections, matching seven years of study. Focused on stepping a witch or wizard through learning each completely and with sensible methodology. Each in their way also pointed out ties to many of the other ten books he created.
The twelfth book took another twenty-seven years to finish. Harry believed it would teach someone with a NEWT in Rituals to be a solid master in three years. The book had three sections to support the idea. This by far had been the most difficult book to write. Certainly, hundreds of books were collected and hundreds of letters to other ritualists were exchanged. It took dozens of trips sneaking out of Britain to conferences, meetings, and simple discussions with other experts in the field over those long years to accomplish the book to his high standards.
By the time the Mastery of Rituals book was completed no one had really "seen" The Boy Who Lived since the end of the war. It made sixty-year-old Harry laugh hearing where people thought he might be hidden or that he was an impostor. Not that he ever let any fairly outrageous tidbits slip out about his own life. No, he'd never do that. He certainly never mentioned nor does not live in a tropical hideaway on top of a mountain in Antarctica with a Viela harem.
Kreacher had passed many years ago, but Bob and Sally Mae were just as enthusiastic about finding books for his ever-growing library as Kreacher was. Resulting in Harry still very rarely leaving his home. They ultimately did have to convert the fourth floor like they had done with the fifth floor. By this time he was sending books out to be copied as there were far more than he could do. Sally Mae managed this aspect of the library wonderfully.
Working with Bob and the American book dealer, Harry found a book publisher in New York. The printer was extremely happy to keep Harry's secrets for producing and selling copies of Harry's twelve books after he had them professionally reviewed. At least two of the reviewers' glowing comments made it into each book.
The success of the series of books was amazing. Harry decided to only sign one hundred sets ever (1,200 books in total) and swore an oath to it. After keeping the first signed set of twelve for himself, he released them to be sold. The auctions for those 1,188 books earned more than he got for the first year of sales. He was shocked by this result.