J.K. Rowling owns Harry Potter... Stupid veritaserum...

Key:
"Speech"
'writing'

Harry had found something special in his room in the Leaky Cauldron. About to leave for his third year at Hogwarts, he had gone over his room to check for anything he had forgotten. Sitting on his bed was a ragged book, opened to a page washed with scribbles, seemingly of another language. He picked it up cautiously, hoping beyond all hope that the tome wasn't cursed like Riddle's diary. He rested his index finger under the book and, in one small motion, flipped the left cover over, revealing a dark but faded red front, with a few symbols printed in blazing gold lettering. Intrigued, Harry suddenly felt the urge to poke it with his wand, and he did so. The symbols shifted and separated as the black haired boy watched on in awe.

'A Marauder's Guide to Magicks.'

Opening the Guide to a random page, Harry noticed a few words in the centre of the page. He flipped to another page, then another, only to find them all identical.

'Take out a quill, but no ink.' He did so, dunking his head into the trunk and pulling it out along a chewed quill. Still not understanding what had possessed him to gnaw on it so savagely, he gazed morosely at the once perfect writing tool, before turning back to the Guide.

'Write what you want to learn about.'

Harry blinked, his quill tip hovering over the parchment. A sense of dread built in his gut; Ginny had been controlled by Riddle's diary, so what would this Guide be able to do?

"Harry?" Mrs. Weasley's voice came from downstairs, saving him from the choice for now. He looked back at the book, which now read 'you should probably go,' closed it and sealed the trunk, stowing the Guide and quill under his arm and planning to find enough time to write in it on the journey to King's Cross.

Halfway through the car ride, Harry elbowed Hermione lightly, giving her a look that told her to pay close attention, and opened the still translated book. the words 'Back again?' faded onto the parchment and Harry wrote.

'Yes, and I would like to learn who wrote this, first of all.' The ink of the Marauders' message followed the path that Harry's quill had taken perfectly, including the jerks caused by Hermione's nudging Harry to try to stop him. She obviously hadn't remembered the diary, but she just as obviously thought that the book was cursed. Harry held a hand in front of her with bated breath, and the black ink split into four colour-coded lines. They snaked into sentences.

'Red is Padfoot,'

'gold is Prongs,'

'silver is Moony,'

'and green is Wormtail.' Harry watched in confusion as Padfoot and Prongs had crossed the green out, as if it wasn't welcome, but the silver hadn't made a move.

The boy-who-lived placed the mangled quill back on the page and the ink balled up on the tip, before he started writing once more. 'What would you recommend that I learn?'

'Prongs and Padfoot agree on transfiguration,' they wrote in the flashing Gryffindor colours. Then, in a fainter gold and red, another line showed. 'This can't be seen by your friend, but you should try to become an Animagus. It's dead useful, but also illegal so don't let anyone know for now. Reopen the book when you are alone.' Harry got the message and shut the book, before glancing at Hermione, who seemed on the verge of lecturing him for possibly endangering his life.

From now on, ANs will be down here, where I can talk about the chapter without spoilers. I will state outright that I have no idea where this is going, apart from the very basic premise, although even this is liable to change as I come up with competing ideas, which I will.

For now, basic Guide mechanics. The marauders write in third person, and cannot write about each others secrets. For example, Wormtail couldn't out moony as a werewolf, and Prongs couldn't out Wormtail's betrayal. The book was created in Hogwarts and as such has Wormtail included, and unable to be removed except by his own magic or extremely powerful magic. However, James and Sirius each found the book after they discovered the betrayal, and had Prongs and Padfoot cross Wormtail's ink out whenever it appeared.