Scott Chacon, one of the guys behind GitHub, has released Pro Git, which, as the name suggests, is a new Git book. He’s made it available under the Creative Commons Attribution-Non Commercial-Share Alike 3.0 license, which is to say it’s free. It cover the basics you’d except, but really shines when it comes to advanced usages, customization, policy, hosting, internals, etc. It’s the resource to reach for when it’s time to loose your amateur status. And did I mention it’s free?
You can show your support by buying a print copy (the author’s link, not mine).