Learning about calisthenics training can seem simple, you just google it right?
Complete Calisthenics - Ashley Kalym
Complete calisthenics was the first book I bought on the subject so it is only natural that it comes up first.
Overcoming Gravity - Steven Low
I actually have the First Edition of this book, which is now on its 2nd.
Becoming a Supple Leopard - DR Kelly Starrett
Becoming a Supple Leopard is not a Calisthenics Book specifically, but it will change the way you train in a different way.
Convict Conditioning - Coach Wade
Convict Conditioning was the second book I bought about calisthenics and, although the exercises it covers are more basic than Complete Calis and Overcoming Grav, it is certainly more inspiring.
Bigger Leaner Stronger - Michael Matthews
Bigger Leaner Stronger is the Fitness Bible.