There are some fun tricks you can use to speed up your mental calculations. Depending on what you're using math for, you cherry-pick the ones that are most useful for you.
On the first-pass read through, I marked sections I wanted to invest time in learning. Then on the second-pass, I'm planning to practice those techniques. (but will probably procrastinate this)

There were moments of frustration however. So many of these tricks have exceptions. Things like:
If the first number ends in a 9 then do this, but if it's a factor then do something else—and you can remember this by remembering my dog chewed the couch Thursday.
It becomes a huge burden after awhile.
As the math scales to increasing difficulty, naturally the tricks start to get complicated. At some point you scratch your head and wonder "this is a shortcut???"

Mine this book for your preferred gems. ( )

This was an entertaining book on how to mentally calculate numbers. Geek alert!

Jim ( )

Contains techniques for lightning fast mental calculation and other things for elementary mathematics and random tricks. For instance, it includes the phonetic memory system for recalling numbers and a way to calculate dates in your head. The techniques listed in the book are a bit counter intuitive, but they work pretty well. The book also contains problems for you to do on your own in order for you to go and improve your abilities at this particular skill.

Also contained within are biographies of other historical lightning calculators and random bits of trivia which makes this book pretty interesting. ( )

I jealous of those who can do mental arithmetic, this book will help me get a leg up. ( )

I kind of don't want anyone else to read this. ( )