The 2+2 "For Advanced Players" series books are all great. Those not yet mentioned include Tournament Poker, Seven Card Stud, and High-Low-Split Poker. The last 2 are co-authored by Ray Zee, who is not only a good writer, but one of the top cash players in the world.
Here are a few lesser-known but very valuable books that feature compilations of short articles: Improve Your Poker by Bob Ciaffone, and Poker Essays vols. 1, 2, and 3 by Mason Malmuth.
Phil Gordon's Little Green Book is also excellent.