The Adventures of Huckleberry Finn by Mark Twain — Written in Hartford during Twain's most productive decade in the house on Farmington Avenue, this is the novel that defined his genius and still defines American fiction. Reading it before visiting the Mark Twain House makes the tour considerably richer.
Uncle Tom's Cabin by Harriet Beecher Stowe — Stowe had already published her most famous novel before moving to Nook Farm, but her Hartford years shaped her later writing and advocacy. The book itself — scandalously controversial in 1852, credited by Lincoln with helping start the Civil War — is still more readable than most 19th-century American fiction.
Hartford: The City and its Origins — The city's role as the insurance capital of America and one of the wealthiest per-capita cities in the late 19th century is genuinely surprising to most visitors. Local historical society publications give good context on how a small river town became one of New England's most important cities.