You've got to feel for Minerva McGonagall after the Battle of Hogwarts, really you do. First putting everything back together and fixing the castle. Dealing with the dead & injured, with grieved parents and the whole hullabaloo of the immediate aftermath.
Then what? Now she's the headmistress. What's waiting for her up that spiral staircase? Everything Snape left behind.
Books, notes, work journals, loose parchment and scraps of paper, even some ripped out of Muggle notebooks or a corner of a napkin. You saw what the Half-Blood Prince's potions text looked like, every reference book and trade journal he owns is probably like that.
He never taught out of the official text, he wrote instructions on the board in potions class. What other formulas did he tweak, change, completely re-write? New potions he'd developed, if only in theory, only on paper.
Snape was a brilliant wizard, whatever his flaws were. Harry would have told her what he'd seen in the pensieve, maybe even let her look herself. So now she knows.
She knows Snape has no living relatives, no next-of-kin. Does she just scrape all of it into a box and shove them away in a storage room somewhere? Throw them out? Burn them? Suddenly Minerva McGonagall finds herself to be the sole keeper of this troublesome man's life work and legacy.
She pushes them aside for a while, it's just too much. Everything has to be put back together. New teachers need to be hired. September rolls around and a new year begins, a whole new batch of firsties to be sorted, and the seventh year dormitories are a little cramped now, but they'll make do.
But maybe every now and then she pulls some of it out, sits down and goes through it. St. Mungo's, of course, gets the healing spells and new or improved formulations for healing potions. That's an easy enough choice.
But the rest? His notes are often dense and written in a kind of personal short-hand though. Or maybe just not always legible. Not all of his scribblings are entirely wholesome, even if many of them were purely intellectual/theoretical exercises. At least she hopes that's all they ever were.
But some of them are quite clever and useful, and if only they could be figured out as practical spells…. It would be a shame to just let it all be lost and forgotten, she feels.
So she shrinks the stack and wanders off to an empty classroom and pulls her wand out of her sleeve and gets to work. It doesn't all come easily. A few backfire spectacularly and leave her with a bruised arse and a bruised ego and she's reminded of just how stubborn he was, how infuriating he could be.
She holds one-sided conversations with the memory of him, sometimes insulting him, sometimes demanding he reveal the answer, shouting at the empty room.
Sometimes wishing things had gone differently. After all, she'd known he was clever. But perhaps not how clever. Why did you have to join that madman as a boy, Severus? Why did you have to die like that? Merlin, what a waste!