Its hard to make suggestions without knowing what you've already read. As a general rule of thumb I would recommend Phil Gordon's Little Green and Little Blue books. They're great for players of any level and extremely readable--a trait which is hard to come by in poker books
. He also just released his Little Gold book which is supposedly excellent and more advanced if you've already read the other two.
If your looking for books that are tournament specific than you can't go wrong with Harrington on Holdem'. Its a 3 book set and will give you more than enough material to study.
Beyond this just read Cardschat. Its a great resource and 100% free!