I have read a lot, and will put in my 2 cents here.
First, I agree with the Phil Gordon books, and HoH Vol 1+2. I enjoyed 3 as well, but that is a matter of taste, and it is a matter of opinion whether the way he suggests playing the hands are relevant for todays online players, but I likes it.
Tommy Angelo : Elements of Poker. IMO a great book.
Sklansky has a number of books. I really liked Tournament poker for advanced players. (Obviously for Tournament play) I also really like The Theory of Poker. It covers almost every form of poker, so is not geared primarily around Hold em, but is an excellent read IMO.
Winning Poker tournaments one hand at a time is a great study on the thought process from the beginning to end of hands at different tourney stages of 3 winning players.
Mastery by George Leonard. Has nothing to do with poker, but deals with setting goals and how to deal with one-step-forward-two-steps-back phases of learning anything. It like a "its the journey, not the destination" kind of philosophy book, but I found it very good, and if you are trying to reach a goal in poker (or anything) it can be a good source of info.