Hopefully both. I use Hibernate on a daily basis, so I wanted to write a book I would pick up and refer to. While we aimed the book first and foremostly at new developers, we wanted to make it useful to developers as a reference. Let me highlight an example of both.
For experienced developers, a useful chapter will probably be the Appendix, "The Complete Hibernate Mapping Catalog", which is designed as quick reference for possible mappings, complete with java classes (with XDoclet tags), ER diagrams, class models and sample mapping documents.
It allows you for example to say "I want a Bidirection Set, what does that mapping look like". So you open the guide and you get everything right there from start to finish.
For newer developers in chapter 7 we discuss Data Access objects. Many more experienced developers have already seen this pattern around, but developers new to Hibernate may not know how to apply it to Hibernate.
We start with a naive implemention of a Hibernate DAO, pointing out it flaws, then slowly refactoring it throughout the chapter, eventually using a Spring based AbstractDao, to demonstrate how much coding that particular project can save you.