I've been deep into building a geocoder the past month. While we may get rid of Solr eventually, it was a great foot in the door to information retrieval. It helps that I have a problem to solve and a deadline, so I'm motivated to read and work through these books. These three texts have been very helpful. The last book is an excellent overview of text processing and some real world problems you may encounter writing your search engine.

Solr 1.4 Enterprise Search Server http://www.amazon.com/Solr-1-4-Enterprise-Search-Server/dp/1...

Programming Collective Intelligence http://www.amazon.com/Programming-Collective-Intelligence-Bu...

Building Search Applications: Lucene, LingPipe, and Gate http://www.amazon.com/Building-Search-Applications-Lucene-Li...