I've been busy like a bee (for anyone curious, even though I know nothing about bees, I've somehow convinced myself that my fall-back job would be to run an apiary), but I've managed to squeeze in a couple chapters of NHibernate in Action. The book was supposed to be out in August, so i expect it'll be out any day now. The authors also happen to have stellar credentials 
I'm far along enough to say that the book is a good read and a great introduction to NHibernate - which is complex enough to merit a couple books. The early chapters, which deal mostly with high level stuff and configuration are by no means mind-blowing entertaining, but getting through them wasn't a chore. Probably because they cover the basic configuration upfront and refer to the reference manual for everything else. I'm not too deep into the rest of it, but so far so good, and skipping ahead at the later chapters I'm glad to see many advanced topics seem to be covered.
If you can't wait, you can get access to an early version ebook for $25.00 via http://www.manning.com/kuate/. Personally, I think $25.00 is overpriced for ANY ebook - but I bought it nonetheless because there really isn't any other alternative.
This is a good compliment to NHibernate's excellent reference manual. Also worth mentioning is that Jimmy Nilsson's Applying Domain-Driven Design and Patterns: With Examples in C# and .NET has a chapter devoted to NHibernate.