Sure there is lots of documentation for Hibernate (and thus for NHibernate too), including books, but the examples are either typically small snippets, and many are outdated and just don't work with NHibernate v0.8. Some people have criticized the small amount of documentation I have for my WilsonORMapper, and rightly so, but I've always included a real example app that goes a long way in getting people started -- and without this I found it quite difficult to get up to speed on NHibernate since I was forced to dig into scattered documents, forums, and just plain old-fashioned trial-and-error in many cases! Here's an example -- NHibernate has multiple ways to model related collections (sets, bags, lists, and maps), which certainly is an advantage if you need flexibility, but just try to figure out which one to use for a many-to-many related collection that you want to be bindable -- the answer is use the bag.
Anyhow, what's intriguing to me is how close both the basic approach and the mapping file syntax is for my WilsonORMapper and NHibernate -- they are remarkably similar and there's very little effort to switch between them. Of course there are certainly differences too -- NHibernate is more flexible if you need that (most don't), while my WilsonORMapper is a little easier (far simpler codebase), supports more databases (including Access), and provides DAL capabilities as well as O/R Mapping. Both mappers have features the other lacks -- but they both have the basic set of features that most people need -- and both work as my example apps demonstrate. What are some other features my WilsonORMapper has? Supports stored procedures, custom collection types, real batch updates/deletes, server-side paging, lazy-loading without keeping "session" open, interface to avoid reflection, null values even in .NET v1.1, and I'm sure there are others I could list.
What about NHibernate you ask? I can't do it much justice to be fair since I'm not that experienced with it, but a few come to mind: more caching options, normalized inheritance, built-in one-to-one, two-way graph syncs, more cascade options, eager load option -- and again there are bound to be more. But the basics are pretty similar (although I prefer my WilsonORMapper of course) -- so if you're an NHibernate fan, then please download and enjoy my "real" NHiberate example app -- and feel free to add your own comments here on either mapper (or others), although please include some real details if you do.