Understanding Git
That is actually one of the better intros I have seen, which surprised me some-what (there are quite a lot of introductions to git now!).  Possibly I've just used git enough by now -- albeit not intensively -- that it was easy to follow, but I completely agree with his contention that you have to understand the repository format to understand git, and he does a good job of of covering that in context.Scott Chacon's gitcasts included a keynote he gave which also has a good explanation of the repository and how it works: http://www.gitcasts.com/posts/railsconf-git-talk.  He has a peepcode book covering the same material, which I bought out of appreciation for gitcasts at the time.