If you're debating the question of whether or not to put an index in your book, err on the side of caution. A book is more usable, and thus more marketable if it is indexed.
Indexing is a specialized skill.
A list is also available in the book, the Literary Market Place , although it is always better to work through a referral. Try your indexer out on a trial basis by having the person do a sample run and set a price beforehand. Remember that the index is usually the last thing to be done because the indexer will need page proofs in order to specify page locations.