Hm, as someone who's finished that book, I don't know that I'd say Haskell Programming from First Principles really teaches much category theory -- it teaches specific applications of a few things as it exists in Haskell.

I'm currently working through Conceptual Mathematics (an intro category theory book), and while the Haskell book has some overlap, it's been extremely minor. The main thing I can think of at the moment is determining the number of arbitrary maps that exist between sets. There is some overlap of things like monoids and functors, but even those have a different, more nuanced treatment in category theory (for example, the relatively simple fact that endomaps are functorial interpretations of the sum monoid was new to me, coming from that haskell book).