What is an index?
An index is a logical mechanism by which a reader can quickly and accurately access an author's information presented in many formats. Usually this takes the form of an alphabetically ordered subject and name index but there are many other types of indexes as well. An index is tailored to fit the needs of the material and the audience.
If a reader cannot find the desired information they will not use the text. Neither the broad categorization of a Table of Contents nor the unfiltered results of a search are designed to anticipate the needs of the reader. A good index will. Not only does it point to the locations of specific data but also offers readers access to the information through general concepts and related terms--quickly and accurately.
***For an interesting search vs. index useability comparison survey see www.tinyurl.com/BNAindexstudy Bloomberg BNA (specializing in legal and business resources) has found that readers found information they were looking for significantly faster and more accurately using an index versus a search function.
Who should index?
Someone who is trained in good indexing practices. It is also preferable that they be a specialist in the field of the text. Professional indexers can bring a fresh perspective to the text and, using dedicated indexing software, can deliver a clean and properly formatted index in a timely manner.