I took a course at UChicago on how to read a book based on Mortimer Adler’s book of the same name, and the takeaway is that the first reading of a book should be a quick one (table of contents, skimming, dipping into pages). Only after a book is shown to have promise are we then to engage in deep reading which not only involves summarizing but also syntopical reading, which is to read other books around the same topic.
One thing that has really helped me is to scribble notes in the margins and to write a précis after every chapter, then a précis on the inside cover of the book to summarize all my chapter précis.
Finally all good books should be read twice or more. Good reading is rereading. This is a hard rule to follow because nobody has time to read the same book twice but the truth is you can’t understand a book deeply on first reading because you don’t have the lay of the land and the benefit of retrospection. A rereading helps you focus on details missed the first time around. To be honest though, very few books meet my bar of my willing to reread them.