This book is quite like a textbook. There is actual mathematics and theorems are proved. I was familiar with most of the material, but it was presented in a very entertaining way. Many proofs very quite slick. Even though there are chapters on arithmetic, computation, algebra, calculus, combinatorics, and probability, I think it is really a logic book. Everything eventually comes back to logic, specifically the ideas of incompleteness, unsolvability, and reverse mathematics. The author's overriding thesis concerns finding the boundary between elementary mathematics and more advanced mathematics. At first, this seemed unimportant and distracting to me, but eventually I saw that the author's aim was to shed light on this topic by means of reverse mathematics, which I find quite fascinating. In fact, I remember that almost 20 years ago Harvey Friedman came to the University of Minnesota where I was studying logic and spoke on the topic. He really made an impression on me.