I want to give him three (or more) math-related books as his birthday present, books that would inspire him, expand his mathematical thinking and introduce him to new concepts. I am also looking for books that won't exceed his current aptitude level too much, lest he might be turned off completely from reading the book. I realize it would be difficult to judge this without knowing him closely, but I hope the information I gave is adequate.
I am not new to HN, but I created a new account specifically to post this question, as I my son is also a HN reader and I don't want tip him off.
I have found a MathOverflow thread (http://mathoverflow.net/questions/8609/favorite-popular-math-book) which gives me some ideas, but I am looking for the collective wisdom of HN readers. Please bear with my indulgence.