[math-fun] Suggestions requested for a recreational math book for a bright 15-year-old

A bright friend of mine just turned 15. They are rather advanced in math for their age and a good chess-player. At age 12 I was ecstatic to receive the first book in Martin Gardner's series of collected columns, around 1959. My friend has not seen anything by Gardner yet, so those are the first thing that comes to mind. Yet I wonder if at this point they might seem somewhat dated. All suggestions and opinions welcome. --Dan ________________________________________________________________________________________ It goes without saying that .