My choice for one single book to recommend (for a beginner or intermediate player) would be Dan Heisman's A Guide to Chess Improvement: The Best of Novice Nook. I've learned more from that than from any other single book I've read. It covers the widest range of topics I've seen, including thought process, how and what to study, tactics and strategy, psychology and time management, and many others.
My other choice -- I know, only supposed to be one recommendation, but I can't resist because it's such a great work -- would be Pachman's single-volume Modern Chess Strategy, or the multi-volume Complete Chess Strategy version if you can find it. I only have the abridged single-volume work, but many people recommend the three-volume version, which has lots more examples than the abridged version. I'm still working my way through this book, but it's done more for my positional understanding and my ability to evaluate positions and come up with promising plans than anything else I've read.
Surprised no mention of Reuben Fine's Books. Chess the Easy Way has much of his excellent The Ideas Behind the Openings. Includes elementary endgame situations (not comparable with his famous endgame tome, Basic Chess Endings, which still gives Sillman and Dvoretsky competition) but plenty for non-reference purposes. And good advice in general. Readable, yet not a lightweight beginner's book, although it starts with the rules. Quite thorough.