Indexes are an essential part of any non-fiction book (and even some notable novels). It provides another way for readers and browsers to find the information they are looking for. This is true for both types of index users. The first type are those who have read your book already, and are hunting for something they remembered reading. The second type of users are potential readers looking to see if your book has the information they need.
My job as an indexer is to make your book more accessible to your readers and potential readers. A good index can actually increase the sales for your book. I have been told many times that both librarians and classroom instructors will often make decisions based on the presence and quality of the indexes in the books they are considering.
The presence or lack of an index is also something that is occasionally mentioned in published reviews of books, particularly in the field of history. It may amuse you to know that one of the regular features in The Indexer, the international journal of indexing, is comprised of quotations from published book reviews that mention indexes, their faults and even when they have been omitted entirely. Reviewers do pay attention to indexes, at least some of the time. This might or might not be a factor in your decision to have your book indexed.
Contact me at firstname.lastname@example.org for more information. I’m looking forward to working with you.