For some strange reason I don't think I've ever read any of the books on any standard mathematics reading list. Of popular maths books, I have read:
* Fermat's Last Theorem, by Simon Singh
* The Equation that couldn't be Solved, by Mario Livio
* Kepler's Conjecture, by George Szpiro
* Poincaré's Prize, by George Szpiro
* Four Colors Suffice, by Robin Wilson
I do, however, have Chaos by James Gleick, but I never finished reading it. For some reason it was not as interesting.