I put together a very readable, beginner friendly book on Hibernate called Hibernate Made Easy. It covers all of the basics, shows you how to get started, and takes you through a variety of very simple and easily reproducible examples that will get you proficient in Hibernate very quickly.
My book isn't intended as a reference though, but as a very easy to read and understand book on the fundamentals. I always suggest that anyone using Java also compliments their library with Data Persistence with Hibernate by King and Bauer. That is the Hibernate reference, and covers practically you could possible imagine in the realm of Hibernate - but the drawback is the extremely thorough coverage sometimes takes away from the use of the book as a "Begginners Tutorial."
By the way, Harnessing Hibernate is also a very recommended resource as well.
Here's a little list of books I recommend for learning Hibernate, and why I like them:
Recommended Books for Learning and Mastering Hibernate
Hibernate really is pretty amazing. Jump into it, and you'll love it.