"The Young Visiters", a novel by a nine-year-old girl written long enough ago that JM Barrie wrote the prologue to the second edition. Here's a quote:
At least Daisy was nine when she wrote it; others have no excuse. Amanda McKittrick Ros was such a bad poet, the Inklings (Tolkien, Lewis, and others) would have competitions to see who could go on reading long enough without breaking up laughing. More recently, sci-fi conventions have done the same with "The Eye of Argon", a fantasy novel so bad that for thirty years nobody would admit to having written the thing.
On a more serious note, there's Henry Darger's lengthy epic "In the Realms of the Unreal"; he was a janitor that suffered from some combination of autism, Tourette's, and schizophrenia. It's in the tens of thousands of pages long, but some of them are illustrations, traced bit by bit out of magazines; it's a sort of sci-fi story about a child-slave rebellion in a world where the largely Catholic earth orbits a much larger planet, populated by horned aliens. I'm really not sure what to think of it. The fact that the illustrations have most of the characters nude (and sometimes the girls have penises) the whole time doesn't help much either.
And, finally, one of my favorite books ever, "English As She Is Spoke". Two gentlemen of Portugal decided there was money to be made writing a Portuguese-to-English dictionary; however, they spoke no English. Luckily, they both spoke passable French, and they had a French-to-English dictionary, and so they went ahead. The resulting travesty against language was the world's first ironic best-seller, and Mark Twain wrote an introduction to a later edition.