I am reading a book called Solr 1.4 Enterprise Search Server now. I have a plan to seriously dive into Solr soon and as of now this is the first and only book about Solr on the market (so one can say it is both the best and the worst book about Solr now). I am not going to give you any review on the book as there are other (more valuable) reviews and related sources. The following are some of them:
Probably the best book review so far (very detailed) has been published by Eric Hatcher at Lucid Imagination blog: http://www.lucidimagination.com/blog/2010/01/11/book-review-solr-packt-book/
I did not know that there is an interesting apache wiki page PacktBook2009 listing missed content.
The most important review of the book for me was Grant Ingersoll's post on his blog: http://lucene.grantingersoll.com/2009/09/24/review-of-solr-1-4-enterprise-search-server-part-1/ By far this is not detailed review and moreover it is just first-part of the review (will the second-part be ever posted?) but it made me want this book.
How about future Solr books?
I think that Otis Gospodnetić (sematext.com) is working on his Solr in Action book. Given that Otis is experienced book author I would expect that this will be a MUST for every Solr fan.
How about other interesting search related books?
I am looking forward several interesting books to be finished, started or at least announced:
Taming Text (this book has been in hibernation for some time - it seems that Grant is very busy)
Mahout in Action (I still don't understand why the scope of this book does not cover full Mahout API. This makes the book unfinished.)
Lucene in Depth (it seems that Jake Mannix is working on a new book about Lucene: An advanced topics book to go beyond Hatcher, Gospodnetic and McCandless' awesome Lucene in Action introductory text. This sounds cool!)