For me biographies are easier, like The Man Who Loved Only Numbers.
Sometimes I make Anki flash cards for interesting facts, it can serve as a motivational factor for continuing on.
I feel non-fiction books are supposed to engaged with; I am supposed to stop for a minute and stare into space turning an idea over in my head, maybe ask or email someone about it or research it online.
I think people who tear through non-fiction books like fiction often have a self-cultivated structure to when and how they read books. They pick a book or topic they're curious about and make sure they apply themselves to it systematically...