The NHibernate project was started back in 2003 by Paul Hatcher, and with the tremendous work done by Mike Doerfler and Sergei Koshcheyev, it has steadily become a mature product, popular with thousands of .NET developers.
NHibernate was originally a port of the incredibly popular Java Hibernate project, and object/relational mapping has been very popular with the Java crowd for many years.
A consequence of this popularity is that Java developers have access to a whole heap of books about Hibernate. In fact, the last time I counted I found 15 books dedicated purely to this single tool. New books on Hibernate and related technologies are still appearing regularly.
Until now, .NET developers have had no such luxury for learning ...