An index is a route map of the names, places, ideas and themes within a document, usually listed from A to Z. Readers use the index to find and return to key information in the text.
Indexes add to the quality of information products and potentially improve sales figures. Book reviews often mention poor quality or missing indexes and potential purchasers use indexes to assess the coverage of a book and to compare books.
Indexing is a specialist skill and it takes a significant amount of time to prepare an index well. It is an analytical process, not a keyword search, and it cannot be adequately replicated by computer software.
Indexers are trained to understand user needs, to think about the range of words readers might use to search a text, and to provide concisely worded entry points to allow easy access to each topic. This includes finding implied concepts that are not directly named by a keyword within the text and which wouldn’t be picked up in keyword searching.
Large topics are further analysed into contextual subheadings so that readers don’t have to work through long lists of page references. Cross-references are carefully constructed to show the relationships between topics.
One of the hardest-to-learn indexing skills is knowing what to leave out of an index. Crowding the index with every instance of a word (likely in a computer-compiled index) is not helpful to the reader. A professional indexer knows when to leave out a reference to a brief mention of a topic that has been covered fully elsewhere in the book, or which doesn’t contain anything useful.
Indexers look for different forms of words that automated word recognition or keyword searching might not pick up, for example information about ‘rams’, ‘ewes’ and ‘lambs’ would all be gathered under the heading ‘sheep’. Indexers provide extra information to guide the reader, for example when a text contains homographs such as pens (animal enclosures) and pens (writing tools), a qualifier is added in parentheses to distinguish between the terms. Indexers pick up information in illustrations and graphics that a text search might miss, and show where material is presented in different formats, for example, by using italic locators for illustrations.
Indexing requires analytical thought which cannot be replicated fully by computers and cannot be picked up quickly by authors trying to index their own books, especially with deadlines looming.
Commissioning a professional indexer to index your publication can save you time and stress, and ensure that your readers will gain the best possible route map to your content.
 Society of Indexers https://www.indexers.org.uk/about-indexing/information-for-publishers/
 Dartnall, Jean. 2008. ‘Are indexes worth it? Evidence from book reviews.’ Learned Publishing, 21 (4), 325-327.