Listen to the Author Instead of Reading the Words
Reading textbooks can be very dull work. An average reader's attention is bound to slip at least ten times a minute and would find himself reading the same content over and over again with no comprehension. More often than not we end up throwing aside the book in frustration and calling it a day.
Being a Physics student, I am expected to read volumes of textbooks on a wide range of topics. The authors of these texts are generally authors capable of packing a lot of meaning into innocent looking sentences and if read casually, one is bound to miss something.
Listening to someone talk about the same content though, can be much fun. Particularly if they are talking about it with excitement. Imagine your friend spiritedly describing his science project to you. You'd be glued to what he has to say. Most of us find it easier to understand things conveyed to us orally than reading about them. Building on this idea, what if you started LISTENING to the author instead of reading his words?
No, I'm not talking about audio books.
While reading, I imagine the author speaking it out loud to me. In my head, I am sitting there with the author, we are having a discussion and he is explaining his idea to me. This way, I pause to question everything he says and listen to every small detail that he gives. I have discussions with him and reread his statements for answers. This works magic for me in my understanding and attention span.
For you, it doesn't necessarily have to be the author himself. You could imagine your favorite teacher or anybody else whom you would love to listen to. You could add as much details as you would like to make it more interesting and productive. The idea is for you to get the maximum out of the content of the book and stay focused for a long time. This is much like what we do when reading fiction. We imagine the way each character looks, imagine their voices and their emotions, though all that's in front of us are words.
Of course, if the content of your textbook itself interests you enough, no one else is needed. You, your mind and the idea are great company in themselves!