As someone who used to use Haskell a lot, I will tell you that learning category theory isn't really going to improve your practical ability to architect Haskell code.

If you're simply curious about the mathematical structures that Haskell's standard typeclasses are named after, then by all means learn category theory.

But if you're trying to improve your ability to use those typeclasses to write real-world software, I recommend "Haskell From First Principles" and/or "Effective Haskell". And get your hands dirty writing actual programs - practice makes perfect.