Playing with Ease is a book about ergonomic technique for the guitar, as well as other instruments. Renowned classical guitarist David Leisner offers an introduction to the basic anatomy of movement, advice on relieving unnecessary tension, pioneering ideas about engaging large muscles, and tips for practicing and concert preparation.
Just the chapter on practicing was worth the cost of the book. Overall emphasis on reducing tension is salutary whether or not you agree with everything in the book (I'm still not sure about his concept of "engaging the large muscles" while playing).