I am quoting from others' works because they said it first, and so much better than I could.
All nonfiction books should have an index to enable readers to find
information quickly and easily. Indexes are tools of accessibility. If they
are poorly made and cannot do the job for which they are intended, they
frustrate the readers. Simply put, a bad index diminishes a book. Even
those publishing houses that require authors to provide their own indexes
recognize that the majority of them lack the objectivity, the training and
the time essential for this vital task.
Barbara Horn, The Effective Editor's Handbook (1997)
The following is an extract from a leaflet published by the UK Society of Indexers, called "Authors and indexers: DIY or hire a professional". You can find a PDF file of the full leaflet on their website, www.indexers.org.uk.
A good indexer understands the readers' needs as well as the subject.
Indexing is rarely a popular task for authors. It takes objectivity, perspective, a sense of proportion and priority, patience, speed, technical training and experience. If you have all these qualities, if you can apply them under the pressure of tight deadlines, and if you would rather index your current book than start writing your next one, you are the best person for the job. If not, you should hire a professional indexer.
But I'm the expert on this subject
The index is meant for the reader, who will often not be an expert and may not be as familiar as you are with the concepts, argument or vocabulary of the subject.
A professional indexer will look at your book from the readers' perspective, anticipating how they will approach the subject. The indexer will analyze the text and, using synonyms, main headings and subheadings, double entry and cross-references ('see' and 'see also'), provide a carefully structured index to guide the reader efficiently back into the main text of the book.
Average rates for relatively uncomplicated manuscripts range from $2 to $2.50 per typeset page. This rate can vary greatly. If you'd like a quote for indexing your manuscript, please contact Anita directly.