I was struck by the category theory section, which starts with two fairly easy books (Cheng and Awodey), a book I've never heard of, and then Mac Lane. Mac Lane

I was struck by the category theory section, which starts with two fairly easy books (Cheng and Awodey), a book I've never heard of, and then Mac Lane. Mac Lane is a fearsome book, which takes a lot of concerted effort and study for me to read. I have had four years of the Cambridge Maths Tripos, so I am (ahem) probably quite a bit more highly trained in mathematics than most readers of this list, and Mac Lane is just barely on the edge of things I can read. It should come with a health warning for its extreme difficulty and abstraction; there's a reason it's in the "Graduate Texts" series.