Pop math book you would love to see written 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.
For some examples of what I mean by "pop math" see this or this post.

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.
 A: There's room for a really well done biography of von Neumann.  The one by MacRae is ok, but not great.

I always thought Von Neumann’s brain indicated that he was from
another species, an evolution beyond man. --Hans A. Bethe

Peter Lax described von Neumann as possessing the "most scintillating intellect of this century."
Von Neumann was the first to rigorously establish a mathematical framework for quantum mechanics.  He played a key role in inventing the computer.  He invented game theory, and later used ideas from game theory in advising US officials at the highest level during the cold war.  He is credited with the equilibrium strategy of mutually assured destruction, providing the deliberately humorous acronym, MAD.

"If you say why not bomb [the Soviets] tomorrow, I say, why not today.
If you say today at five o’clock, I say why not one o’clock?". -- von Neumann

There's ample material for biographies here.
Edit: A new biography called The Man from the Future: The Visionary Life of John von Neumann by Ananyo Bhattacharya has been published.
A: The Mathematical Gamer: How deep mathematics arises in video games.  Many competitive video games can be analyzed mathematically and the mathematics could get pretty deep.  Easily some probability problems, calculus, linear programming (real-time strategy games like starcraft), pursuit-evasion (tag-based games), calculus of variations (geodesics in racing games).
Also various puzzles lend themselves to talking about abstract mathematical ideas.  For instance, there's group theory in Peg solitaire, and linear algebra in Fiver.  It could be a fun way to introduce these abstact concepts.
A: A pop math could do more justice to the work of the ancient mathematicians such as Sieve of Eratosthenes. Modern ways of presentation using applets embedded in a web page would really bring their work to life, and it would help explain such artifacts as base 60 time telling.
