I found the Psychology of Poker interesting, but not entirely sure it helped my game. I think understanding psychology in general though would be +EV and a big help in getting tells and hand reading.

ToP and SSH are the nutz. I have a bunch of books, but those are the ones I reread. I'm getting Super System 2 for Xmas, so we'll see how that holds up.

HPFAP is good, but I don't think I'm yet at the level it helps my game much. Mostly when I read it and try to apply it, it fails because the players aren't good enough to apply the concepts very well.