One of the easiest ways to to write a short paper on the book. It doesn't need to be a proper academic review, simply a few bullet points and/or paragraphs where you sum up the book, its arguments, and its evidence.

I usually write about 1-3 pages this way of bullet points with a couple sentences about each point. Also, if something jumps out at me such as a footnote or a quotation, I mark the page number for future reference. I hand write these, as plenty of research has shown writing to be better than typing for retention. When I'm done, I tear it out of the pad I'm writing on, fold it up, and put them inside the front cover of the book.

Not only does it help to organize your own thoughts on the book, but it helps you retain what you've just read. And, in the event you need to refer back to the book in the future, you already have notes that outline the work. I've found this to work great when pulling books for annotated bibliographies later on.