I just finished reading Hibernate Search in Action, and I loved it. I should point out that I was the porter of Hibernate Search to NHibernate Search, so I had some previous expertise in the topic. In addition to that, I approached this book at an angle completely orthogonal to the expected audience. Unlike most "in Action" books, I did not intend to make immediate use of the code and approaches suggested in the book. Instead, I looked to the book as a way to deepen my understanding of the tool and how it works.
I am impressed, massively so, that it did so well in this regard for someone who has gone through the entire source code of the project several times.
I'll not bore you with the actual details, you can get the actual content summary of off the site. From my perspective, after reading this book I know that I am going to take a completely different approach for most complex search scenarios, and I think that I have the practical theoretical knowledge to deal with it.
I highly recommend the book if you actually need to deal with Hibernate Search, but I would recommend it to people who are not using it, because it contains some important eye opening concepts if you are not used to full text search tools capabilities. As a nice bonus, I was able to take the information in the book and use it to discuss a problem the customer was having, ending up with something that I consider far superior of the solution that they currently employ.
It is not out yet, and I reviewed a non final copy, but you can order the PDF right now, and just reading the freely available first chapter is valuable in itself.