A well-narrated story of the path to the solution of the "Four Color Theorem". Robin Wilson explains the theorem and the various attempts to solve it in a clear, lucid manner. The mathematics is simple, and aided by nice maps or map-like illustrations. In between the mathematics, the author weaves in interesting facts and anecdotes about the mathematicians who concerned themselves with the problem.
The path to the solution is intriguing, winding over and around the various hills of different branches of mathematics. I haven't gotten to the end yet. Can't wait to get my hands on the book again :)