I don't usually like these types of big list questions, but I think this is actually fairly important as far as education, informing the public about what we do, and getting people excited about math goes. There were some pop math books that exposed me to great ideas and essentially made me want to become a mathematician.
What circle of ideas, biography of a mathematician, conjecture, or subject would you like to see as a popular math book for non-mathematicians?
Answers might be in the form of a theoretical title plus a brief description. For example,
Bezout's Theorem: a history of how the quest to understand how two curves intersect led to the birth of modern algebraic geometry.