A Executive Summary on this topic can be viewed and saved here.
Here's a list of links to other references and resources:
The American Society for Indexing's Digital Trends Task Force has an informational page with many links to other resources.
The Socitey of Indexers (UK) has a Publishing Technology Group informational page.
Bloomberg BNA's Using Indexes Online Study can be viewed here (see page 2).
More about Indexing vs. Searching at this site.
International Digital Publishing Forum's EPUB3 Indexes Specification.