MIT Technology Review (http://www.technologyreview.com/articles/rnb_052104.asp) runs a story of how Researchers from Palo Alto Research Center (PARC) have invented an index system that generates custom indexes and tables of contents organized according to a set of keywords that the user enters to describe the concepts she is interested in. The custom entries are drawn from the original manually-generated book index entries.

So when you enter a set of keywords, the system narrows down its index of thousands of entries to a single page's worth of a few dozen entries.