The Little Book of Talent features a bunch of really interesting things to think about as a coach, in a very consumable fashion.
Wooden on Leadership is full of legendary coach John Wooden's insights into coaching and leadership. It's a book I definitely recommend you put on your reading list.
The Coaching Habit by Michael Bungay Stanier is more a management book than a coaching one, but it provides an interesting set of questions you can use working with your...
If you haven't read it already, you definitely want to make Daniel Coyle's The Talent Code the next book on your reading list. It could really impact your coaching.
A Fresh Season is Terry Pettit's second coaching related book. Like his first, it's a series of essays and other short pieces covering an array of subjects.
A Program with Purpose by Johan Dulfur is a book that shares insights and experience about volleyball team program building and development
While Living on the Volcano is about soccer managers, the profiles it includes provides and interesting set of perspectives on coaching.
It's not a volleyball book, but Fake Fundamentals by Brian McCormick is the type of book coaches of all sports should read to make them think about what they do.