According to professional indexer, John Noble, "The book might be worth reading but, if there is no index – how do you find the information that you want?" Below is a link to a good article that may help you decide whether or not your book would benefit from having an index created by a professional indexer. It's a short read, and I encourage you to take the time to read it. Pay special attention to John's comments regarding the value of indexes!
"Interview with Fellow of the Society of Indexers; John Noble"
~from Angel Candelario Rodriguez's Tech Writer News Blog
Thanks to fellow indexer, Catherine Barr, for sharing this article!