I've always liked A List Apart. It's very user-friendly and readable, even to people like me who aren't great at understanding code.

And CSS ZenGarden is what made me want to switch from Internet Explorer to Firefox. IE doesn't render CSS as well as Firefox does.

I've found forums are much more useful than textbooks when it comes to solving web design problems. Each time I make my own website I seem to run into new problems, and someone on a forum always seems to know the answer.