Recently recommended to me by a good friend, I'd never heard of David L. Hough's well-regarded guide to safer riding, Proficient Motorcycling. A very fun read for a foggy Saturday afternoon.
Full of wonderful commonsense advice and examples, Hough never loses track of the one thing that always keeps my attention when riding: it is the things you can not see in time that kill you. I've learned some new things, like approaching a curb or curb-sized bump at a 45deg angle or greater provides for much greater stability. His descriptions of how a motorcycle balances, traction works and all the general physics are the clearest and simplest I've found.
It isn't a skills refresher course, but I'm glad to have read it.