What is an index?
Apart from celebrity biographies (which anyway are perhaps nearer fiction), most non-fiction books have or should have an index. Found at the back of the book, an index is an alphabetical guide to the elements within the preceding pages that the reader might wish to look for (and to do so with ease and alacrity) in order to have a sense of the texture of the text, or to see whether an expected reference might be contained in the text, or to seek a remembered occurrence of a particular allusion, or to gain a sense of how elements of the book are linked.
A search engine could do that.
Even the most sophisticated programs fail to read with a human sensibility. Professional indexers bring to the job a feel for text (from long experience of indexing and writing and/or working with copy), an understanding of what readers look for (and what they don't), discrimination concerning important elements and passing distractions, sympathy for the field in which the book's author strives, and the judgment to see where cross-references and alternative expressions will guide the reader to the appropriate pages and related references. Unlike programs, indexers are flexible, analytical, judicious and able to put themselves in the shoes of the reader.
I can surely index my own book.
Maybe you can. But you need a particular mindset. You need to be able to step back from the text and see the wood for the trees. Unless the book is very specialised, you need to see the make-up of the index with the eyes of both the general and the particular reader simultaneously. And like all professions, indexing has exacting standards. Professional indexers are graduates of a rigorous training course and are expected to hone and maintain their skills throughout their working lives. As with any other skilled job, the only guarantee of good quality is the professionalism of the supplier.
But it's just a list of words so where's the skill?
Unlike a computer program or a non-specialist, the trained and knowledgeable indexer will arbitrate between ambiguous concepts, distinguish between homonyms, draw intellectual links and bring together related notions. Some random examples: knowing which President Roosevelt is intended; separating 'principal' and 'principle'; recognising which sense of 'bow' is meant; cross-referencing 'councils' to 'local government'; seeing that the Napoleon alluded to is the one in Animal Farm... And, unlike a find facility in a program, an indexer won't waste your time by telling you that Cher turns up in Thatcher.
And then the indexer will want paying.
You bet. You're getting a professional service of high quality. Click here for the Society of Indexers' recommended Fees. Rates are negotiable within practicable limits. My preference is for an agreement based on word-rate.