> To actually learn it, Mac Lane & Saunders "Categories for the Working Mathematician." seems to enjoy continued popularity.
This is the best way I've found so far, and I had been semi-supplementing with MacLane and Birkhoff's "Algebra", just to get a feeling for the vocabulary; bringing myself from the space I typically mathematically exist in, to the space of category theory. Nothing else really has come close in composing the correct thought direction. It is dense, but I enjoy it very much. It is very beautiful math.
I have always sworn by Borceux' Handbook of Categorical Algebra. It progresses smoothly all the way from first principles up to (if you buy all three volumes) sheaves and topoi, but it is kind of expensive.
This is the best way I've found so far, and I had been semi-supplementing with MacLane and Birkhoff's "Algebra", just to get a feeling for the vocabulary; bringing myself from the space I typically mathematically exist in, to the space of category theory. Nothing else really has come close in composing the correct thought direction. It is dense, but I enjoy it very much. It is very beautiful math.