Austin Kleon makes a simple, but important point, about how to become a writer:

I believe that the first step towards becoming a writer is becoming a reader, but the next step is becoming a reader with a pencil. When you underline and circle and jot down your questions and argue in the margins, you’re existing in this interesting middle ground between reader and writer:
Kleon has previously recommended Mortimer J. Adler and Charles Van Doren's How to Read a Book, which I bought last time he mentioned it. Ironically enough, it's sitting on my bookshelf, unread. Anyway, he quotes Adler and Van Doren as saying:
Full ownership of a book only comes when you have made it a part of yourself, and the best way to make yourself a part of it — which comes to the same thing — is by writing in it. Why is marking a book indispensable to reading it? First, it keeps you awake — not merely conscious, but wide awake. Second, reading, if it is active, is thinking, and thinking tends to express itself in words, spoken or written. The person who says he knows what he thinks but cannot express it usually does not know what he thinks. Third, writing your reactions down helps you to remember the thoughts of the author. Reading a book should be a conversation between you and the author….Marking a book is literally an expression of your differences or your agreements…It is the highest respect you can pay him.
I read a lot of non-fiction books on my e-reader*, so the equivalent of that for me is Thought Shrapnel, I guess...

Source: Austin Kleon

* Note: I left my old e-reader on the flight home from our holiday. I took the opportunity to upgrade to the bq Cervantes 4, which I bought from Amazon Spain.