Just finished an excellent book on ADHD. "Attention Deficit Hyperactivity Disorder (in Adults and Children): The Latest Assessment and Treatment Strategies" by C. Keith Conners and Juliet L. Jett.
It's clinical, but lots of good definitions and research. And while it claims to be the "latest" it is from 1999 and I imagine there is a boatload of newer information. However I did find it useful.