I personally read "Category Theory for Computer Scientists." - http://www.amazon.com/Category-Computer-Scientists-Foundatio...

It was pretty rough. Reactions by page:

1-2 "This is pretty easy." 3-5 "Okay, I can see how this is getting harder..." 6-... "My mind is blown."

The real difficulty is that it dives into mathematical notation that I'm not used to reading fairly quickly. I'm still slogging away at it in my spare time.