This week's giveaway is in the EJB and other Java EE Technologies forum. We're giving away four copies of EJB 3 in Action and have Debu Panda, Reza Rahman, Ryan Cuprak, and Michael Remijan on-line! See this thread for details.
I made the experience that the book is quite good to read and start with hibernate, sometimes look something specific up. But since the hibernate version is often ahead I used the online documentation instead.
There is THE hibernate book from Gavin King and Christian Bauer but there are also other books about Hibernate. Depends on what you are looking for... just go to amazon and look it up.