I think there are multiple ways to do it. These are what I can think of.

* Re-read the books

* Summarize the book in one sentence or a short paragraph

* Create or visualize the idea or important points in the books, like https://sketches.sachachua.com/static/2022-08-03-01%20Four%2...

Last but not least, I recommend to read a book called "How to Read a Book" by Mortimer J. Adler. Different books have different strategies to read.