Indexes help sell books. Readers scan an index to see what the book is about. The science of indexing guarantees that indexes are thorough, elegant and easy to scan.
The art of indexing relies on experience with language and with how people think. Under what term will a reader choose to look in an index? Sometimes it is a word that does not even appear in the text! Very often, technical terminology counts. How many terms will they try, before they give up?
Use the space you have to give your readers a sense of what the book is about. Make it easy for them to find what they are after.