Did you know that it's possible for e-books to have functional indexes? It's not common - which is a shame - but it can certainly be done.
A common argument against indexing e-books is that readers can just use the search function to find what they're looking for. This isn't a great option, though, for the same reason that computers can't generate usable indexes. Say you're reading a book about the Beatles, and you want to see what the author has to say about the song "Help!". Searching for the word "help" won't do you any good, since it will find every single instance of that word in the book. There's no way to make it find only the text discussing the song.
On the other hand, a hyperlinked index at the end of the book solves that problem perfectly. It works just like a printed index, except that instead of page numbers, you'll see hyperlinked locators. All you'll have to do is tap on the locators and you'll go straight to the relevant section of the book.
My most recent project was writing the index for Listening to The Who: Album by Album, Song by Song by rock critic Benjamin Marlin. Even this simple index of song and album titles will help readers find the information they're looking for quickly and easily. Instead of trying to remember which album "Eminence Front" appears on, they can just find it in the index and tap on the locator to go straight to it.
See how easy that is? If you're writing an e-book, get in touch with me to see about indexing it. Your readers will love having a functional index to refer to!