Sorry for the shameless self-promotion, but I'm just about done with Head First Wordpress (http://www.amazon.com/Head-First-WordPress-Brain-Friendly-Cr...). It covers Wordpress 3.0 (although not the MU integration) and I've tried to make it more about using Wordpress as a content management system. This is exciting news from the WP team and there are a ton of cool new features in this release--my favorite being the menu admin and custom header images.