You're right, save that search is nothing without a great index and it happens I have made one... If you want to know how an analog index works search for more info on what is a Zettelkasten, as my own 'system' is very close to that. Using that index I can find any book I've read (and my notes about it), any notion I've ever written about (be it a book or mere vague ideas),and I can find them in a matter of an instant. No matter how old it is, how short or how... flimsy it can be compared to other and much thicker parts of my notes and research.
Edit: https://zettelkasten.de/overview/ (it's in English). To get a glimpse of a 100% analog version (like mine), check Luhmann's work (the one who formalized the idea, an idea people had been using centuries before he formalized it): https://niklas-luhmann-archiv.de/nachlass/zettelkasten (this one is in German).
At my humble level, my system has become my second brain as well as a precious companion allowing for deeper conversations with... a better/smarter version of myself.
I tried digital mind you, I started using a computer in the very early 80s and must have tried a lot of those digital tools (even contributing money and suggestions to a bunch of of them), but as far as I'm concerned nothing beats pen and index cards and the most essential part linking them all: indexes (plural, as I use two different ones).