The book I'm recommending is pretty far from a textbook. Rather it is an Exercises in Style type book that explores many different ways to express a mathematical proof of the same simple fact. Some of the proofs are whimsical, and others offer genuine insight. Many cover the ways that such mathematical discussions would have taken place in earlier historical periods.

In other words, 99 Variations is more like a jaunt through mathematics culture rather than a book that focuses on actual mathematical content.

I'll take a look at this book, though, because it looks quite good!