If I were limited to a single book, it would be Don Quixote, which, since it is a book about books and reading, contains every book before it and every book after it. And as a metafiction bonus, Part Two contains and comments on Part One.

The category of "book" on these lists rarely includes drama (Euripides, Shakespeare...) or long poems (Virgil, Dante, Akhmatova...) or essays (Montaigne...). The compilers of such lists seem to love to read, but they apparently didn't have teachers who taught them what reading really means. Reading isn't just about the pleasure you get from escaping into a book--it's about the pleasure you get from discovering more about your world.