I've only done this once or twice for books I've read, but a comment here from a post many years ago suggested taking an index card per chapter and writing a series of questions or ideas on one side, and the answers on the other. Then at the end of the book you can review and see if you remember what you thought were important points and see if you remember the 'answers'.

I found it really useful, but it took willpower to do this, and I soon fell out of the habit. Il have to try it again.