Library of Congress Subject Headings, back-of-the-book indexing and web indexing.
Library of Congress Subject Headings (LCSH)
The American Society for Indexing is the body advises on professional indexing in the US. Their website is a small treasure trove of resources and advice on indexing. They also list online reference resources, including online subject thesauri.
Similar resources and training materials for aspiring indexers are available from the Society of Indexers, based in the UK.
The American Society for Indexing SIG (Special Interest Group) - Digital Publications Indexing provides detailed resources on indexing for online portals, directories and websites.
Spool, J.M. (01.09.1997). Why on-site searching stinks. User interface engineering [online]. An interesting, if dated, article comparing the effectiveness of Web page indexing against keyword searches using on-site search engines
A blog posting from 2005 following Spool (1997), that claimed little had changed in the relative efficiency of searching indexed and non-indexed Web sites.