[bluGill]

Not really. An index is also a list of ideas you should search for. Search for a synonym and control-f fails, but the index will have a "see also" for that, or worst case lets you scan for interesting words without reading the whole book. The index will also leave out all the places where a word happens to be used but are not useful to someone searching for the term.

Of course a good index is hard (read expensive) to write and so many books didn't have good indexes.

[eesmith]

I got "A City on Mars: Can We Settle Space, Should We Settle Space, and Have We Really Thought This Through?" for Christmas, by the Weinersmiths.

The index is so good I've shared my happiness about it several times.