If the author (or anybody) really wants to absorb knowledge from books, it's simple and only takes two steps:

1. Take notes while reading, which usually also has the side-effect that you have to re-read passage around your note taking

2. Convert notes to Flashcards utilising SRS (spaced repetition systems like Anki)

Thus all the info you noted and found interesting will go into your long(er)-term memory.

But that's not why we read most books now, is it?