I'm going to make a few suggestions here, but you need to know that I'm the author of Hibernate Made Easy, so my opinions on that book will be very biased.
I personally believe there are three really key books to get on Hibernate, two of which have been released in the last month or so, namely, Harnessing Hibernate by James Elliot, and of course, my book, Hibernate Made Easy. The other book on Hibernate that everyone should have is Java Persistence with Hibernate, as was already recommended.
If you're using mapping files, Harnessing Hibernate would probably be an excellent book for ramping up with Hibernate. It is pretty much a continuation of The Hibernate Handbook, which has always been on of the most popular hibernate books on the market.
The goal of my book, Hibernate Made Easy, is to really introduce people to Hibernate and show people how easy it is to get going with Hibernate. I use Hibernate and JPA annotations in the book.
Java Persistence with Hibernate is the reference tome. It's extremely thorough, covers just about everything, and really, every Hibernate developer should have a copy. It does cover everything, and as such, you could imagine that it may not be the easiest book to learn from. But once you start to understand Hibernate, and you really need to do some advanced things with Hibernate, well, you'll be thankful you have it by your side.
So, if you can only afford one book, I'd suggest Java Persistence with Hibernate. If you can afford 3, then I'd suggest adding mine and Harnessing Hibernate. If you can afford more, well, you're a much richer man than me.
There are a number of good reviews on Hibernate books on amazon. I think that for the most part, the reviews fairly accurately reflect the value the various books provide. Do some research on that front when it comes time to decide.
Check out my signature links for some more thorough reviews on Hibernate book currently available on the market.