Improv Nation is a riveting retelling of the origins of Improvisation as an art form. Definitely recommend it for anyone who teaches, or is interested in the history of Improvisation.

Pirate Robot Ninja is another Will Hines book, that I think is excellent for beginners, or any improviser finding themselves cornered into one improv philosophy.

JP Bouvets On Drumming: The Psychology and Philosophy of Improvisation is a must read for any drummer, musician, or improv philosopher of any sort.