This may come as a surprise, but many authors (especially those from outside of the academic world) know little about indexing. They do not see the value of adding an index to their books, especially after learning that this addition comes with a price tag. Below are a few reasons why they should give their doubts a second thought:
Books with an index sell better. This was discovered by book traders in early modern England. Believe it or not, in the 21st century book buyers are still making their purchasing decisions, often unconsciously, based on the presence of an index. This is equally true for individual buyers, as well as vendors, and acquisition librarians.
Books with an index look more attractive. Indexing is an art, not a technique. Just like creative writing, an index could be not only informative, but also beautiful. It creates order and structure, revealing the main subjects of the narrative, and guiding readers straight to the point, and it does so in an effective and intelligent manner.
Books with an index speak with authority. An index is like a certificate of authenticity. It testifies that a book is an object of intrinsic value, and that it is going to stay in the realm of knowledge for a very long time.