If you have time, Steve Awodey's book "Category Theory" is both beginner-friendly and helpful. You can get a PDF of an early pre-print online. He covers monads towards the end. If you are an experienced Haskell programmer, I've seen a few "Category Theory for Haskell programmers" tutorials around. That all said, I don't think that knowledge of Category Theory is a prerequisite to effective programming in Haskell (or any other language that features monads).