I also have The Man Who Loved Only Numbers and Elliptic Tales on my to-read list once I wrap up Zero.
If you're looking for something else, "Journey Through Genius" is absolutely the best semi-pop-science Math book. It's premise is that it goes through about 10 of the most interesting proofs in mathematics, giving their history and context, then the actual proof.
It does what few other "pop" books attempt - it actually gives the real proof, taking you through all the steps. If you work it, you come away actually knowing how to prove the various things there. More interestingly, it gives the original historic proof, which is not what you always encounter. It even tends to explain the problems with the original proof, if any. And since it gives the history, you learn a lot.
It's by far my favorite non-textbook math book, and I can't recommend it more highly if you're into that sort of thing.