Generally I've found more advanced material requires a lot of "steeping" in my mind before it clicks. I was literally in my 4th year of college before Calculus finally actually clicked in my brain, this book seems to have the same issue. My first couple times I just didn't understand what I was reading, and I put it down (for many years actually, time flies). When I picked it up this time I honestly don't know what my issue was with comprehending it before. In the intervening years I did a much deeper dive into using Common Lisp as my hobby language, and I think that's helped a lot. What I'm reading connects better with the practice I've had in using a Lisp. Similar to calculus, I had spent a couple years applying it (not just studying it) and when I went back and reexamined some basic materials everything just fell into place in my mind.