“Can't you just use a computer to write the index?” While it is true that computer software can be used to generate a concordance, or list of terms and the pages where terms are found in a given text, only human analysis of a text can produce an index that points readers directly to the information they seek.
It is the professional indexer's job to recognize key concepts in a text (whether explicitly or implicitly stated), to identify connections between concepts, and to separate substantive discussions from passing mentions of a topic. The indexer then synthesizes this information, writing appropriate entries and cross references so that all readers can quickly find the information they are seeking.
A well-written index adds immensely to the usefulness and potential sales of a text. Many readers will check the index when deciding whether to purchase a book. A poorly-written index (or the absence of an index) can greatly frustrate readers and may affect buying decisions. A professional index adds value to a text.
If you are an author who must provide an index to your publisher, consider the value of your time when deciding whether it is worth hiring an indexer. By the time page proofs are ready for indexing, the prospect of having to write the index can be burdensome. Hiring a professional removes the stress associated with producing a properly constructed and formatted index within time frames that are often very short. Professional indexers are accustomed to meeting detailed publishing specifications and tight production deadlines.