Secrets of Mental Math by Benjamin and Shermer is good reading even if you don't do all the exercises.  There are lots of simple secrets that are worth knowing, but this is NOT in the book.  When I first learned math all my teachers were women.  When I nine and didn't do well they said not to worry, GIRLS BRAINS WERE DIFFERENT AND THEY NEVER DID WELL IN MATH.  When I got older I was taught binary, hex, octal, radix points etc. by a man teacher and was great at it.  It was then I realized I was good at math, but my women teachers were not good at teaching it.  Why?  Because when they were about nine they had been told the same thing they told me, girls can't do math.  This book is worth reading for any adult, but when you're finished give it to any nine year old you can find, especially girls.  If you have children do yourself a favor and give them this book.