If you are considering indexing for the Web, The Art of Indexing may be helpful but should not serve as your only source for learning about the fundamental principles for indexing books. Instead, I would recommend a variety of resources. That is the primary task of any index. Indexing is a complex decision process involving perceptual discrimination, concept formation, and problem solving. An incomplete index is almost as useless as no index.
Completeness is indispensable.
Include every major topic discussed in the document. Cross-reference necessary terms. Give the reader all the means to find a topic. The key to developing a good index is to list topics in the same manner in which the reader will attempt to find them. Indexing utilities are not a replacement for a professional indexer.