This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates 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.