Although I hate writing on books, I agree that writing annotating a book is an excellent way of highlighting and helping to memorize the most relevant sentences of a text. In order to solve to ruin or not to ruin a book I recently adopted an annotation technique that not only avoids writing on the book that I am reading but it also allows to keep a compact summary of the main ideas that I took from it.
My method is based on the use of small coloured post-its (50x38mm) that I start by spreading randomly across the book. While reading, and whenever I find a key phrase or idea, I look for the closest post-it and I move it to the page that I am reading and I write on it the phrase or idea that I want to remember. I also always write the book page number in the post-it bottom right corner, for later reference.
The post-it annotation also helps to locate definitions or other important concepts previously introduced that only later I realise that I didn’t understood completely
Although I personally believe that the SQ3R (Scan, Question, Read, Recite, Review) reading technique is too slow, I usually scan the book while placing the empty post-its and the written notes help the review process that I always do after finishing reading a book.
However this method does not end with the review process. Before returning the book to its shelf, I take out every post-its with the annotations and I stick them in one or two empty sheets of paper. I do it in order by following the page number marked in each post-it.
This way I have a summarized version of the book that allows me to latter explore even further some of the ideas, be reading the pages indicated in the post-its. The post-it collection also gives a general idea of the structure and contents of the full book.
The last step is to archive the book summary and in my case it corresponds to the scanning of the sheets of paper with the post-its and to store the resulting PDF file in a folder inside my laptop. The physical sheets of paper are stored in my archive in the event of some type of disaster.
The result of the compression process can be seen in my workstation screen.