Comment by doug_durham

Good indices are built atop a taxonomy that is then used extensively to list related taxonomic terms. This will give you direct hierarchical terms (loosely maps to what I guess you refer to as by subject) but also related terms. A good indexer will also exercise judgement and check with the author if certain terms are related and in what way.

Let me give you an example of a high-quality index entry from the Software Architecture in Practice (Bass et al. 2021) [1]:

Availability

analytic model space, 259

analyzing, 255–259

broker pattern, 240

calculations, 259

CAP theorem, 523

CIA approach, 147

cloud, 521

design checklist, 96–98

detect faults tactic, 87–91

general scenario, 85–86

introduction, 79–81

planning for failure, 82–85

prevent faults tactic, 94–95

recover-from-faults tactics, 91–94

summary, 98–99

tactics overview, 87

As you see, it lists a number of taxonomic terms that are merely related to each other and you might not think about Ctrl+F-ing for them unless you already want to search for them. You may come to this entry knowing about CAP and navigate away to analytic model space, 259.

[1]: https://www.goodreads.com/book/show/14786083-software-archit...

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.