The Shoebox Project is a novel-length marauder-era Harry Potter fanfic co-written (and illustrated) by LadyJaida and Dorkorific. In it, we follow the Marauders (Remus Lupin, Peter Pettigrew, Sirius Black, and James Potter), and more specifically Remus and Sirius through their final two years at Hogwarts. The story is told mainly through Remus and Sirius' perspectives, and the overarching plot of the story is a romantic one between Remus and Sirius, but the story also does a great job of fleshing out the friendship between the marauders, the character of Peter Pettigrew, the relationship between Lily Evans and James Potter, and the effect of Voldemort's rise to power on the lives of students at Hogwarts and recent graduates from Hogwarts. The story is alternately told through conventional third-person narrative, letters between Sirius and Remus (and sometimes James) over the summer, notes passed in class, and photographs and items that have supposedly been collected in the shoebox that the fic is named after.
I absolutely love this fic, and can say without equivocation that it is by far the most well-written fanfiction that I have ever read. The characterization of the marauders is masterfully done, and the references to the original books are both subtle and heart-wrenching. Examples: Sirius is horrible at divination because he only ever sees 'drapery' in his crystal ball; James has a passing thought that he would die to protect Lily when they first start dating. The writing style for both of our main viewpoint characters is thoughtful, deep, rich with sensory details, and distinctly in character. On top of that, this story made me cry, especially at James' and Sirius' reactions to the death of James' parents, who were also Sirius' adopted parents after his own parents disowned him.
The characters here are so well drawn, and so well sourced from what we know of them as adults in the original books that I have a hard time remembering, off the top of my head, that the events and relationships of these books aren't actually canon. Lily and James Potter's relationship is wonderfully realized in this story in a way that makes me care about them more than that the original books ever did. The scene in the bathroom (not what you're thinking) is especially adorable. Sirius and Remus are a wonderful mismatched pairing going through puberty and the self-realization that they have a thing for each other in a time where it would still have been stigmatized. Peter is pitiful and understandable in a way that lets you hate him even as you feel pity for him.
Plus, Lily Evans has tea with Professor McGonagall every Wednesday, which is a small detail that brings me immeasurable joy.
The story is occasionally silly, especially in the first third. Sirius' spelling and punctuation are atrocious, but that's obviously on purpose. There are, however, other errors in the text that are not on purpose; never enough to detract from the experience of the story, but enough to remind you that what you're reading is, in fact, fanfiction. If you're as enchanted as I was, you'll barely notice.
The Shoebox Project will always be Harry Potter canon for me. I absolutely adore it.