Great Teams is a book that does a really good job combining sports and business insights into a resource with a lot of good concepts and ideas.
Nikolai Karpol is perhaps best known for his yelling, but that masks a whole lot of coaching success. This book shares his ways of thinking about things.
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.
Dream Like a Champion - Wins, Losses, and Leadership the Nebraska Volleyball Way by Brandon Vogel and John Cook is an interesting view into the mentality of one of today's most successful coaches.
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 players and teams.
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