The Godfather. Not great literature, but I have read it probably 15 times and enjoyed it just as much every time - most recently just this past Spring.

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If you let me have five, I would add: 2. The Lord of the Rings (the set, of course, including The Hobbit) 3. Shogun 4. Everything by Robert A. Heinlein (impossible to choose just one; I would have trouble trying to narrow it down to twenty) 5. Everything by Larry Niven, (same apology.)

But wait - that leaves out Michener's many superb works, and the entire Ender universe of Orson Scott Card, and everything by Scalzi, and Herman Wouk. And single gems like Watership Down, To Kill A Mockingbird, The Once and Future King, Jurassic Park. Then I would add I Claudius/Claudius the God, the Hannibal Lector books of Thomas Harris, the Jack Ryan books of Tom Clancy...

Okay, okay, I admit that I suck at this game. It's a dumb game, imho. Thanks for asking.