Scott Chacon is one of the guys behind github -- a hosting site for projects managed by Git distributed version control system. As such, Scott is extremely competent in all things Git, and he wrote a book (or should I say the book) on Git. The book is called Pro Git. I've been using Git for some time now, so I couldn't wait to get my hands on a printed copy of the book. (I'm old-fashioned that way -- I prefer reading paper books.)
Pro Git is published by Apress. On 250 odd pages, Scott manages to bring Git across in a very vivid way. Lots of sample sessions and a huge number of figures make Git come alive and fun to use! (Only at one point while reading the book, did I think: "why is he telling me that?", but it can't have been too bad, because upon writing this, I can't find the place.)
Pro Git starts off easily enough, but it does so at a fast pace, for which I was grateful: you get past the basics on page 45, which means the book gets you set up quickly, so that you can start taking Git for a spin.
Chapter 3 is called "Git Branching", and the thirty pages explain all you ever wanted to know (as well as all you never wanted to know ) about branching. This part is hardcore, but Scott explains this with lots of diagrams, making it easier for us to follow.
One of the best chapters in the book, for me, is called "Git on the server". Scott explains the different methods of setting up Git so as to be able to collaborate.
If you use Git or intend to (and I'd certainly recommend you look at it), I very warmly recommend Scott Chacon's Pro Git: it's the best companion you'll have for Git.