Books (sorta): Sid Meier's Alpha Centauri instruction manual
I know what you're thinking. How can an instruction manual for a PC game be counted as a book? Well, "Sid Meier's Alpha Centauri" (known by the delightful acronym "SMAC") is no ordinary PC game, and its manual is no ordinary manual.
SMAC is a turn-based strategy game from the same guys that brought you the "Civilization" series (in fact, it's part of the series in all but name, and many of its ideas creep up in later Civs). The Civilization games, though, could rely on human history to furnish a great setting for your empire-building. SMAC had to provide its own backstory, because it's set in a future where humankind has managed to colonize a fictional planet orbiting Alpha Centauri, Earth's closest stellar neighbor.
The manual is about the size of a paperback novel - nearly 250 pages. While much of this space is occupied by gameplay mechanics and tables, there are fascinating sections describing alien life - including entire hypothetical ecosystems and astronomical speculation (heck, they even have a "Suggested Reading" section that includes stuff like Carl Sagan's "Pale Blue Dot"). Even cooler are the designer notes in the appendix, which serve as sort of a "director's commentary" for the game and give real insight into how tough it is to make a science-fiction world believable and yet alien. I don't suppose you'll ever find the manual in a bookstore, but if you do ever come across it, it's worth a look.