Indexing is a simple concept, but difficult to execute properly. Too often indexing is an afterthought, compiled without much planning. Authors and publishers sometimes begrudge the expenditure involved, but it is an investment in reader satisfaction. A well constructed index adds immeasurably to the value of a non-fiction book, making search and navigation simple and satisfying. An index automatically generated by a word processor or layout package is useless without the hand of an experienced indexer.
The Book Show on ABC Radio National recently ran an interesting program on the finer points of indexing.
If you are considering engaging an indexer, visit The Australian and New Zealand Society of Indexers and take advantage of their directory of available indexers. All people listed have completed the appropriate training.