Hannibal is a town that has fed more novels than it has produced — and there is one obvious reading list.
The Adventures of Tom Sawyer (Mark Twain, 1876). Twain's most popular novel and the one most directly built from his Hannibal childhood. The fence, the cave, the island, and the river are all here.
The Adventures of Huckleberry Finn (Mark Twain, 1884). The darker and greater of the Twain river novels, beginning in a Hannibal-shaped town called St Petersburg before drifting south down the Mississippi.
Life on the Mississippi (Mark Twain, 1883). Memoir of Twain's years as a steamboat pilot, with a long section revisiting Hannibal in the 1880s. The best of his non-fiction.
The Bandit of Hell's Bend (Edgar Rice Burroughs, 1925). Not set in Hannibal, but written in the rooms Burroughs rented at the Mark Twain Hotel — a footnote, but a charming one for visitors.