1. Highlight the passages that are important along the way
2. After each chapter I try to summarize its important points and any challenges I have to them.
3. Over multiple chapters, I have a section at the top "Key Ideas" with the top 5-10 points over the course of the book.
4. After the book is done, I edit the doc to include a "My Thoughts" section and gather all my critical thoughts into one place. This is probably the most valuable.
Overall, even if I don't remember the details of a book, now I have a collection of summaries that I can pull up to remind myself about each book if it comes up again later and I need a refresher.
As a bonus, it's super easy to write a review of the book afterward because you have all the pieces almost already in place.