Are you under the impression that authors create indexes? Because generally, they do not. Specialists do it (because it's way harder than it looks). Some academic publishers might ask the author to pay for that work; bigger publishers might absorb it. But I can't imagine any respectable publisher accepting an index from an author who did it themselves.

Sorry your comment contains "surprising", I am not going to read it.

> My rule: no index, no sale.

With e-books though, why do you even need an index when you can just search?

What percentage of "good" books include an index?

For non-fiction, I think I have always unconsciously followed the GP rule, because I don't remember any book, good or bad, that lacked one.

Fiction, of course, rarely gets one.

I can't think of any fiction that has an index. Do you have examples?

Answering my own question: the American Society for Indexers has a list. https://www.asindexing.org/about-indexing/indexes-and-indexe...

Life is too short to read 99% books.