https://www.scientificamerican.com/article/a-learning-secret...
http://www.npr.org/2016/04/17/474525392/attention-students-p...
If I don't have paper nearby, I write in the margins and dogear the pages, and then refer back to the book to extract the information later. I keep a few pens and pencils in my pocket at all times.
Reading on a phone, I get the ability to look up unfamiliar words immediately, add them to my spaced recognition tool if they seem useful, add quotes to my quote file, and arrange notes on new things learned either per book, or on a per project basis. In addition, taking notes like this they are still available over the years, and backed up in multiple places, whereas the paper notes I have taken are either filed away somewhere or lost during one of my moves.