Or just fiction with a bunch of characters?

I prefer paper books, but I can definitely understand someone reading a good novel, especially a long one... or a novel in a series of novels... wanting the ability to do a quick search to look up some character or event they've forgotten about.

I'm not a Frank Herbert fan, but I know his books are pretty dense and there's a fair number of them, so if you're a Frank Herbert fan, especially one reading the series for the first time, having the ability to do a quick search back to the third book when you're reading the fifth would be pretty helpful.

Another good example would be Tolkien. I assume you don't think Tolkien is bad fiction.

I am non-native English speaker and the ability to lookup words easily was very helpful for me.