This charming book explains why math olympiad puzzles are fun, and gives 15 year old insights ( in two senses --- most of the text was written by the 15 year old Terence Tao, but revised with some additional good exercises 15 years later by 30 year old Fields medalist Terence Tao.)  The style is chatty, and the advice and worked examples are very good and do not require any math beyond pre-calculus.  The level of difficulty is just right for would be high school math competitors, and adults with some math who enjoy a mild challenge.