Hard to pinpoint. In terms of entertainment books, it'll probably come down to a tie between Douglas Adams' Last Chance to See and Walter Moers' The City of Dre

Hard to pinpoint. In terms of entertainment books, it'll probably come down to a tie between Douglas Adams' Last Chance to See and Walter Moers' The City of Dreaming Books, or maybe something by Terry Pratchett (although I couldn't single out a book there, either). The reason for those is just that I can't imagine ever being bored reading them. Maybe also Flowers for Algernon (Daniel Keyes) for being a really creative concept (the way the story is told) and for being a wonderful example on how interesting modest sci-fi can be without resorting to "LAZERZ!!!". Edit: Another contestant: Friedrich Dürrenmatt's The Physicists. Grim yet humorful. I generally really like Dürrenmatt's dark endings. Edit 2: And another one: Measuring the World by Daniel Kehlmann, specifically for how well he got me to understand Gauss' weird reactions and motives without going into his work at all. Before I've read it, I would have sworn that a book that tries to present one of the greatest mathematicians would need loads of math in it.