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.
Legacy, by James Kerr, has some structural elements I didn't like and didn't quite match expectations, but definitely has value.
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.
While it isn't about volleyball, The Inner Game of Tennis is a classic book about the athlete mind and how to progress toward optimal performance.
Here's my review of Kathy DeBoer's book Gender & Competition, which takes a look at how men and women view things and act in very different ways.