Physical books are great. Internet goes out or other devices die and you need to complete work. These are reasons I like to have book references.

Also, one of my favorite things to do when reading is writing in my margins. When I figure something out or find something interesting I like to write it in my own words in the margins, and then if I have to reference again, I have my own words and explanation in the margins to help myself understand faster and better.

I also like to add sticky notes for the same purpose.