The index is an essential part of any non-fiction book, providing another way for readers and browsers to find the information they are looking for. This is true for both those who have read your book already, and are hunting for something they remembered reading and for the reader who is looking to see if your book has the information they are looking for. I know that in my student days, I frequently used the index to determine the usefulness of a book to my needs when writing term papers.
As an indexer, I can make your book more accessible to your readers and potential readers. A good index can actually increase the sales for your book. Both librarians and classroom instructors have been known to make decisions based on the presence and quality of the indexes in the books they are considering. Not to mention the casual reader who is checking to see if the information they want is actually there before they buy or borrow your book.
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, which is the international journal of indexing, is a column of quotations from book reviews that mention indexes, their faults and even when they have been omitted entirely. 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.