You should construct evergreen (permanent) notes based on concepts, not related to a source (e.g. a book) or an author.

Your mental models are compression functions. You make them more powerful by trying to use them on new information. Are you able to compress the new information with an already acquired function? Yes, then you've discovered an analogous concept across two different sources. Sort of? Then maybe there's an important difference, or maybe it's a clue that your compression function needs updating. And finally, no? Then perhaps this is an indication that you need to construct a new mental model – a new compression function.

Whereas Dr. Sönke Ahrens in How to Make Smart Notes seemed to be saying that the writing of a permanent note (~evergreen note) is a unit of knowledge work with predictable effort & time investment (as well as searching for relevant notes), Andy emphasizes only the note searching activity in this context.