The Joy of X

 The Joy of X: A Guided Tour of Math From One to Infinityby Steven Strogatz

This was a lot of fun. I got a bit lost at times but forged ahead and came out of each chapter knowing something I didn't before. 

There are 30 chapters, with titles that hint at the author's playful approach to math:

1. Fish to Infinity - an introduction to numbers 

3. The Enemy of My Enemy - the disturbing concept of subtraction

5. Division and Its Discontents

8. Finding Your Roots - complex numbers

10. Working Your Quads - the quadraic formula

11. Power Tools - the function of functions

16. Take It To The Limit - the power of the infinite (calculus)

20. Loves Me, Loves Me Not - differential equations

23. Chances Are - the improbable thrills of probablility theory

25. The Loneliest Numbers - prime numbers

27. Twist and Shout - playing with mobius strips and music boxes

Some of that sounded pretty intimidating to me, and it was, but even if I didn't grasp all the finer points, just getting the broader concepts into my head was invigorating. 

I don't know if I'll ever use what I learned, but it doesn't matter. I had fun trying to figure it out, it was entertaining reading, and it was good mental exercise. I'll keep it on my shelf and am sure I'll be referring to it again.