No one outdoes Martin Gardner in writing about mathematics in an understandable way. He is also a very modest and helpful man, in my communications with him he has always been willing to give credit to others, sometimes when it seemed clear that it was his.
This book contains some of the best puzzles that Gardner has encountered and of course, he describes them in his own clear style that makes them so easy to understand. Solutions are included; the level of difficulty is such that the middle school student will be able to follow them. Many of them are of the "Aha!" variety, where the solution is obvious once you encounter it.