How do you make sense of your library of books/readings etc.?

Physical and/or digital. Books, articles, or anything else you might want to reference. How do you make sense of all the resources you've gathered?

Beyond the usual sorting by "Author/Date" and by topics (eg Tech, Psychology), I'm interested to hear about some of the strategies, frameworks, philosophies etc that you have figured out in terms of organizing your library?

Eg. the groupings you use, particular categories/tags (eg "classics"), etc.