I know this sounds like the doting father, but my 9-years old son is pretty good at maths (all right, math). He's at least 2 years ahead of his age in math (and also reading and writing, come to that). Please can anyone suggest some fun-to-read basic math books for him? I'm not a mathematician (though I do a fair amount of statistics in my job) nor is my wife, and we're running out of things to suggest. He can do the basics of algebra, geometry, stats, etc, already, and he actually *loves* doing math problems.