Even if an eBook has a search facility, an index will greatly enhance the accessibility of the content - see here the advantages of a professional index over basic search facilities.
An eBook index can provide active links to references. There are two main types of eBook: those with a fixed layout and page numbers (e.g. pdf files) and those with reflowable pages, where the page numbers change. For the latter, index entries will link to a specific paragraph or point in the text. There are various ways of embedding and hyperlinking index entries to achieve this.
If you are interested in having an active index in an eBook, please contact me to explore how this might be achieved for your text.