It's about using rocks to learn numbers and their properties. Rocks even help in 'proving' a couple of these properties (example: the sum of the first N odd numbers is a perfect square: 1 + 3 = 4, 1 + 3 + 5 = 9, etc).
It's all absolutely fascinating. Strogatz has this great ability to make mathematical ideas sing and dance in your head!
No excerpts here. Just go read it!