I finished this 'beta' book today, it took me a while because I have the bad habit of reading multiple books at the same time. For an NHibernate virgin like myself the book was a very good introduction. It covers everything you need to know to get started with release 1.2.1 version, although nothing is inserted (yet ?? ) for the upcoming 2.0 release. I hope the final version will include some chapters covering the new features, especially the event/listener model seems to be interesting. If it doesn't the book might already be outdated upon its release in August.
The thing I was missing was more in depth information about extending the functionality of NHibernate. We had an issue for instance with the interceptors which you can read all about here, but the book did not provide a lot of information about.
My overall conclusion is positive because it was a nice intro, but if you already have some experience with NHibernate the book might be too 'easy' for you.