I'm going back and forth between a few right now:
Impro: Improvisation and the Theatre, by Keith Johnstone. This is a very interesting book, because the teachings can be applied to so many more subjects in life besides acting.
Facing Violence: Preparing for the Unexpected, by Rory Miller.
Training for Sudden Violence: 72 Practical Deills, by Rory Miller.
Principles-Baseqd Instruction for Self-Defense (and maybe life), by Rory Miller.
Obviously, I like the books by Rory Miller. I also like Marc MacYoung, Kris Wilder, and others' books, which tackle similar subjects. Even if you've trained martial arts for decades, there is always much to learn, and happily, these go far beyond martial arts "styles" and into the true heart of self-defense, including mental, emotional and legal ramifications.
Adventures in Cryptozoology, Vol. 1, by Richard Freeman.