ADD often involves focusing on that which interests and an inability to focus on that which does not.
Personally, when I am in the middle of something interesting, I hyper-focus to the exclusion of noticing people walking around the room and occasionally don’t even notice them talking to me. When I am in the middle of something uninteresting (like most conversations) then I have a hard time paying attention long enough for the speaker to even finish a sentence, and often have to ask them to repeat themselves after I count the cracks in the wall. I can spend hours reading Neal Stephenson or an RPG rulebook, but can’t finish most magazine/newspaper articles that are far shorter.
We all learn differently, and some of us far differently from most. My wife and I are almost opposite as she cannot see pictures in her head very well at all whereas I am like @Kayak8; a visual thinker who often has little/no use for words. That also meant that we both needed different types of special treatment in school in order to handle a curriculum designed for normal people.
Also if I don’t see how something is relevant and have no interest in it, then there is nothing that can be done to make me understand unless/until you can convince me that it’s something I actually want to know. (If I need to know but still don’t want to, then it ain’t happening. Threats of failing tests or getting fired don’t work with me.) Since your book is an assignment and likely not something you actually want to read, odds are that you will always find something to distract you.