A very useful and in-depth book,
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This review is from: Lucene in Action (Paperback)
Lucene is a very in-depth and interesting technology that is used by a lot of large companies (Twitter, CNET etc). This book does a very good job of introducing you to Lucene and then going into detail about how it works.
It covers analysing, indexing, searching and sorting in depth and gives clear code samples.