So many books! Chess becomes all about remembering similar positions - practice and study to understand.
In the public domain, Lasker's Manual of Chess is a classic. I saw a study group use that as a textbook. Capablanca's Chess Fundamentals starts at a lower level, but sometimes leaves proofs to the reader - for instance, tells you to be ready to swap one advantage for another - which advantage, again?
John Watson cleared the bases with "Secrets of Modern Chess Strategy" and "Chess Strategy in Action", which simply dumbfounded the pundits when they came out. Between them, I prodded him with an email suggesting that understanding nuances in positions, and how small differences could lead to big changes was a key.
I haven't bought many chess books lately, but openings books are a dime a dozen and "Basic Chess Endings" is an updated and enduring classic.
It depends on what your current level is and what your relative strengths and weaknesses are. Almost everyone should practice chess tactics, but maybe online puzzles are as effective as books for this. Gambit <a href="http://www.gambitbooks.com/" rel="nofollow">http://www.gambitbooks.com/</a> is a good publisher.