While I have close to 400 books, the following 5 IMHO are the most critical (and 4 of the 5 I've read cover-to-cover, the 2nd one I've skipped a few parts but are getting back to them):
1) The Inner Game of Chess by Andrew Soltis
2) Fundamental Chess Endings by two German guys (can't recall names)
3) Bishop v Knight: The Verdict by Steve Mayer
4) Forcing Chess Moves by I forget his name
5) Chess Lessons by Viktor Popov
The Amateur’s Mind by – Jeremy Silman, A great book for correcting misconceptions and developing strategies
500 Master Games of Chess – Dr. Startkower & J. Dumony. A great book for learning openings as well as the middle game strategies and end games associated with each opening. I enjoy playing the older lines that are presented in this book,
1000 Checkmate Combinations – Victor Henkin
Tactics Time – Tim Brennan & Anthea Carson
Chess Explained or Starting Out on which ever opening I am studding