Taking into consideration that NHibernate has been available for some time I thought that there would be a book available already. A search on amazon turned out just one that apparently has been delayed (Manning). Meanwhile, it looks that there seems to be coming many books on Microsoft Entity Framework. Is there any significance in that?
As far as I know you've answered your own question, though I didn't realize it was delayed. I bought an Hibernate book back in the day when I was learning NHibernate and it was about 99% the same, just ignore the Java JPA stuff.
The same goes for Hibernate tutorials, though there are a number of good NHibernate tutorials to be found via Google...
You can read the ebook of NHibernate in Action with the early access of manning. They will send you a print copy when it comes out if you want.
UPDATE: "This book has just gone to print, and the final eBook is available.", says the author in the comment below
Jimmy Nilsson's book "Applying Domain-Driven Design and Patterns" from AW Press has chapters on NHibernate.
Manning's previous book on Java's Hibernate, "Hibernate in Action" is still very valuable. It will help you grasp querying in Hibernate, which is applicable to NHibernate, too.
NHibernate in Action, currently in early access programme. It covers only NHibernate 1.2 though
EDIT: There is a new book on NHibernate coming up and it covers 2.x version. The like is here
When I started working on NHibernate, I used the document it comes with. It's sufficient to get good level of understanding. Also there are many blogs that will walk you thru of many scenarios. I have a few tutorials on my blog www.sherazblog.com. Also check out the FAQ on http://blogs.hibernatingrhinos.com/nhibernate/Default.aspx. Don't bother reading Jimmy Nilson's book if you are planning to learn Nhibernate. It's a great book to understand DDD. use http://groups.google.com/group/nhusers for Users group. Cheers.
Packt's NHIbernate title "NHibernate 2 Beginners Guide" is out, and it covers NHibernate 2.x and fluent NHibernate.
There is also a free video series called Summer of NHibernate, and also a video series at TekPub, but that one isn't free.