What are the essential resources for learning the fundamentals of LEGO building?
I'd particularly recommend The Unoffical LEGO Builder's Guide. It covers all of the basics and gets into many of the more complex building techniques.

If you're interested in getting right into the details, The Unofficial LEGO Advanced Building Techniques Guide by Didier Enjary covers a wide variety of building techniques and is freely available. I don't know a lot about this document, but I first came across it on The Brothers Brick.