Bartosz Milewski is writing a category theory tutorial, which is in progress:

http://bartoszmilewski.com/2014/10/28/category-theory-for-pr...

I found this one quite approachable:

https://hseeberger.wordpress.com/2010/11/25/introduction-to-...

For category theory constructs in a Haskell context, I think the Typeclassopedia [0] is the best thing I've found.

[0] https://wiki.haskell.org/Typeclassopedia