I've read both of the Super Systems. I was very impressed with Jennifer Harman's limit section in Super System Vol. 2. The books are straight forward and very easy to read unlike that of Sklansky.

I've read Sklansky's "Theory of Poker" and "Small Stakes Hold Em" which are both excellent reads. These are excellent guides to the learn the math side of the game and calculating odds. I found the writing to be quite boring and it reminded me way too much of one of my college math books. Lots of good info though so if you can stomach it, read it.

Hellmuth's book didn't impress me at all. All the talk about animals was rather lame.

Winning Low Limit Hold Em by Lee Jones in a great book. He has a few chapters on SNG strategy that improved my game tenfold. I already knew most of the information that he wrote about but he did a great job of organizing my thoughts for me. Definitely a great read!

Harrington on Hold Em has probably done the most for my game. The book makes you think as it presents you with quiz like situations that make you think. If you are serious about improving your game, definitely pick up both volumes.

This is always an interesting topic of conversation.  I second virtually everything said.  I haven't picked up Supersystem 2 yet so I don't have an opinion on it.  IMO, Harrington is one of the most definitive work on hold'em on the market.  I'm constantly going back over sections because the more I play (particularly live tournaments), the more I can relate to the material.

I'd also recommend Caro's Book of Poker Tells.  I'm not sure a lot of it is as relevant as Caro thinks it is, but definitely good info.  Not long ago, I was playing a tournament and picked up pre-flop tells on the 3 players to my left that were right out of the book.  Having a consistent read was like being in position 4 hands in a row.  I controlled that table all the way to the money.