This is a very good question, and one I’ve struggled with a whole bunch. I’ve begun tagging them to make searching easier (e.g. “History (Analog)” and “History (Digital)” are separate categories), but really I should also begin labeling them with something like conceptual difficulty? It’s a hard thing to gauge, and also a task beyond my current capabilities as I’ve probably read no more than 75 of the books out of nearly 500. But I would love to hear suggestions and happily invite collaborators if anyone sees a book on the list they’ve read and would like to somehow rate or provide feedback on!