I have started reading "What is the name of this book" by Raymond Smullyan. There are lots of the puzzles which have become classic and appear in this book. Seems really good. The other one that I have got is "Liar's paradox and the tower of Hanoi". This is more of an introduction and I am loving it. I had seen riddle of sphinx mentioned at many places and it was in this book that I finally read the whole story. It's written in pretty laid back manner.