Yesterday a new excellent book about NHibernate has been published by www.packtpub.com. (Disclosure: I have been a reviewer thus consider that when reading this post)
This is so far the best and most complete book I ever held in my hands. It covers NHibernate 3.0 and thus is really up to date. The author has done a tremendous job in covering all the important aspects of interest when implementing business relevant solutions. The book is very focused and gives a step by step introduction into all the necessary details to get a solution up an running. No space is wasted for (potentially) unimportant details.
One of the things I like most is that the book/the author does not have a data centric view but is more domain focused.