Secrets of Mental Math, by Arthur Benjamin and Michael Shermer. New York: Three Rivers Press, 2006. paperback, 278 pps.
Secrets of Mental Math is subtitled The Mathemagician's Guide to Lightening Calculation and Amazing Math Tricks. And indeed, the stuff in this book is amazing. The authors illustrate the tricks and they also explain why the tricks work. From mental calculations involving a variety of operations to memorizing numbers to guesstimation, the style is easy to read and the explanations are easy to follow. Answers to the problems can be found in the back of the book as well as a section on how mathematics can help us overcome subjective biases that plague us.
I do a bit of this book each night and indeed, I do find my confidence growing along with the speed of my calculations. Recommended for teens on up.