Sklansky - Dan Harrington's Course Books - Bill Chen's "Mathematics of Poker"
Doyle's Super System was great but it is a bit outdated IMHO. Definitely read it just because it was the first of its kind but the books that have really helped me the most are Sklansky's "Theory of Poker," Harrington's Course Books (which quizzes to test your understanding) and Bill Chen's "Mathematics of Poker". The Chen book gets you to thinking in terms of ranges (i.e. what range of cards does yoru opponent likely have), something I believe is absolutely essential for any current poker player.
Aside from poker books, I would advise anything on game theory. Game theory teaches you how to, in the words of Jay-Z, "move in a room full of vultures." (in fact, it teaches you how to become the vulture).