1. Before starting, take a fresh sheet of paper and enumerate the chapter numbers of the book. Each on a separate line.
2. After finishing each chapter: stop reading, contemplate the main message and try to pen it down at the corresponding chapter numbers of step 1. Focus on writing it in your own words / rephrasing. Key question for chapters >1: what information did I forget to mention in my microsummary until now? Include that in the new summary, don't edit the old one.
3. Turn the page of your sheet after completing the book. Now write a (max) 1-page summary of the book without looking at your other notes.
4. Compare the summary with your notes. Satisfied? Ready! Not satisfied? Get another sheet and rewrite your 1-page summary, taking extra care to mention the missed points of the first summary.
My experience is that you don't really need the chapter notes while writing the summary, since it is now part of your long-term memory because of the writing.
Bonus: 1-page, personal summary of key points in the book you just read. If you forgot the points of a book, take it off the shelf and consult your 1-page summary.
YMMV but it works for me.