In Delve Inside the Lucene Indexing Mechanism, author Deng Peng Zhou provides an introduction to Lucene's indexing mechanism.

Some other Lucene resources:

• Lucene is a talk Doug Cutting (Lucene's creator) gave at the University of Pisa in 2004.
• Introduction to Text Indexing with Apache Jakarta Lucene, by Otis Gospodnetic, covers much the same ground as Deng Peng Zhou's article, but also discusses analyzers and field types.
• Did You Mean: Lucene?, by Tom White, shows how to add a suggestion facility to Lucene searches.
• I Love Lucene, by Dion Almaer, gives a case study of using Lucene to implement search on theserverside.com.
• Lucene in Action, by Eric Hatcher, is a 456 page book published in 2004 by Manning Publications covering Lucene in depth.