There are actually a couple sample chapters available. One is the
Getting Started chapter and the other is on
Using Queries.
In terms of what the book offers above what other books offer I was the co-spec lead and co-author of the spec, so it offers the most insightful and complete coverage of EJB 3.0 JPA available, with lots of examples.
I wrote an FAQ on the book
here that describes who the book is intended for, etc.
Hope this helps,
-Mike