Do you actually want to read them, firstly? I can read “hard” books just fine when they’re on a topic I’m genuinely interested in and I actually want to read the book. But if I force myself to read a “hard” book on something I don’t care about, it’ll be painful.

I always highlight and annotate non-fiction with any thoughts/comments/etc, and if it’s a work I expect to find particularly valuable, I also take separate notes. Usually these are digital notes in the note-taking app of my choice (currently Notesnook), but for some books I get a physical notebook. It also helps motivate me if I get some nice highlighters and pens and take pretty notes.