The Little Book of Talent features a bunch of really interesting things to think about as a coach, in a very consumable fashion.
The Art of Volleyball Hitting by John L. Bowman is a quite thorough discussion of hitting aimed at players, but I think you can find far better resources.
Coaching Volleyball for Dummies has some good content for new coaches, but a lot of the material when it comes to actual coaching is problematic.
The Brain Always Wins by John Sullivan and Chris Parker could be quite beneficial to both your coaching and your own personal health and development.
From the Ashes, by Frank Zaccari, combines business concepts with some history from the University of Washington volleyball program.
Peak - Secrets from the New Science of Expertise helps you understand what the concept of deliberate practice means and how to apply it.
Here is a report of what I saw and heard when we hosted a Gold Medal Squared coaching clinic for the staff at Charleston Academy.
Barking Up the Wrong Tree by Eric Barker is an interesting combination of stories and research that could help you both personally and as a coach.