The Adventures of Huckleberry Finn
Hemingway said all modern American literature comes from this one book, and the claim has held up. Twain handed the novel over to a poor, unschooled boy's voice and let the American vernacular carry the whole thing, dialect and all, which no serious writer had dared before 1884. Underneath the river idyll runs a furious account of slavery and the lies a society tells itself, climaxing when Huck decides he'd rather go to hell than betray a friend. It has been banned, taught, and argued over ever since, and the arguments are the point.
For thirty chapters this is the great American novel; then Tom Sawyer shows up at the Phelps farm and turns Jim's freedom into a boy's prank, complete with rope ladders and fake escapes, while Twain plays along. The ending betrays the book's own moral discovery. Add a racial epithet on nearly every page, and teaching it has become its own genre of difficulty.
The case for it and the rest of the canon open with Pro.
