I’ve always just ploughed (or slogged) through particularly long and challenging books in one go, the same as I would for anything. I’d love to have a better “active reading” strategy though, and one that I actually stick to. “Steelmanning” an argument sounds like a great tool for that.
“Steelman” the argument, reworking it so that you find it as convincing and clear as you can possibly make it.
— A Note On Reading Difficult Books by Brad DeLong