on Just Publishing Advice:

It’s easy to add a table of contents to an ebook. But how do you add an index to a digital book?

For print books, you can create an index easily and automatically with most word processors. It will be in alphabetical order with page references.

But an ebook doesn’t have page numbers, so you need to create a hyperlinked index.

When a reader clicks on an ebook index entry, it will take them to the word or phrase in the text.