This is mainly for math teachers, but I'll try to list some of the more general books I like first:
  • Kagan's Cooperative Learning (go directly to their web site to get the latest version)
  • Every Minute Counts - great book on how to teach math class. A bit old now, but many ideas are still useful.
  • Proofiness - great, readable book on numbers in our lives. Mostly applicable to statistics, but makes you want to understand math in general better.
  • Drunkard's Walk - great, readable book on randomness
  • 7 Habit's of Highly Effective People (I love this book. I mostly have listened to it on audio, but I've listened to it again and again. One day I'll get it :)