The book is directed at Java developers interested in learning Hibernate, but considerring the similarities between Hibernate and NHibernate, it just as useful for .Net developers who wants to learn how to use NHibernate.
The concepts transfer very easily from one framework to the other, and I've only found two places where NHibernate differed from Hibernate (and one of them got fixed since then :-D)
The book starts with a general view on O/R mappers and the advantages of using them, then moves on to the history of Hibernate (which wasn't quite of interest to me). Then it gets to the fun stuff, starting from a simple exmaple, the books shows you the basics of mapping classes to database, working with persistant objects, transaction, concurrency and caching.
After that, you get advance mapping concets and a discussion about efficency which I found quite useful.
The most interesting part of the book, and the most disappointing for me, was the chapter that talked about how to write applications. Most of Java application are targeted for the web, so the discussion centered around patterns for the web. And far less on local applications. Personally, I develop WinForms applications, and rarely ASP.Net applications, other people experiance vary, of course.
Over all, a really good book.