if you love these puzzles, check out these books with excellent low - aha:! questions...

This book is the BIBLE for technical interviews.  They cover EVERYTHING from the programming questions to the open ended questions to the brain teasers.  Written by two Stanford grads, this book is the one book you HAVE to read if you are trying to bone up for a technical interview.  It covers all the common programming questions (linked lists, strings, etc) and most of the common brain puzzle questions (similar to the ones on this site).  If you only have time to read one, read this one.  Its inexpensive and a quick read. 

If people that loved brain teasers all belonged to the same church, it would have to be the church of Gardner.  Martin Gardner is the grandfather of EXCELLENT brain teaser puzzles that never go out of style.  His writings and puzzles are timeless.  His writing style is wonderful.   If you haven't heard of him, then you don't realize that a majority of the puzzles you hear are in some way related to him and his books.

This books has all kinds of historical and ancient puzzles as well as some modern ones too! It goes all through history from ancient kings to Godel's Incompleteness Theorem. Superb book which is quite different than the normal "see answer on page 100" kind of books that are usually out there.

Hofstadter weaves everything together in this wonderfully entertaining book. Not for the faint of mind! Its long and its mind bending and it will make you think. If you have a logical background (computer science, cognitive science, philosophy, etc) this book will enthrall you! Even better, I used to think that since I was so into logic and math, I had no time for music and art. But Hofstadter will show you the patterns and logic that can exist in the world of music and art. Everything meshes in this book.