There are a lot of poker books out there now, and most of them have at least some value in generally outlining how to play solid, aggressive poker live or online. Phil Gordon's set is a little different from most books, and in a good way.
Gordon's emphasis isn't just on how to play medium pocket pairs, or how to calculate pot odds; he is interested in the whole experience of being a poker player, of living the poker lifestyle and basing your life choices around that kind of an ethic. He will cover starting hand requirements, yes, but he also wants the reader to ask himself important questions about the kind of person/player he really is, wants to be, or has the stomach for, particularly in the Black Book. I can't think of another poker author who has approached the subject from this angle.
The Green Book gets more into strategy, and the Blue Book is all strategy, tactics, and sample hands to go through with Phil, and they are all well worth reading, studying, and rereading. Along the way there is a lot of relatively lighthearted poker banter, some name-dropping, and good charm and humor; Phil Gordon is a very entertaining story teller and writer. But the heart of it is the analysis and the attitude, and both shine through with insight and value. There are a lot of hands reviewed where Gordon gives names of his opponents, how they played, how he played and why, and of course he harpoons Hellmuth more than once, as who wouldn't given the chance? It's all done with wit, humor, and penetrating analysis.
Gordon's books are not the heavy lifting of Sklansky or the workbooks of Harrington. Nonetheless they are valuable learning tools, and a reminder that especially in poker there is no single perfect way to play. Add that to his optimistic and invigorating attitude towards life in general, and these books are well worth your time and money.