It's not often I start a book review with glowing praise. This time, I will: if you use the git version control system , or are thinking of using git in the future, get this book. It's excellent.
Pro Git available online (or git clone the book's source from http://github.com/progit ). This means that you can read it for free before considering a purchase. Indeed, that's where I started. However, I highly recommend the dead tree version. Apress ' production quality is excellent and the paper copy is definitely a valuable thing to have.
The book is an excellent introduction to using git; it's perfect for newbies, and a good reference for existing users. It starts from first priniciples. That is, it describes what git is, and what a distributed version control system is. It briefly introduces version control in general, but that is really prerequiste information.
The text is well paced, and very clearly written. The examples are well chosen and the coverage of git's facilities is broad.
The author starts with installing/configuring git and outlines the basic git principles. He covers basic operations (check in, clone, viewing logs, tagging). Then he moves onto git's crowning glory: branching and merging. This potentially tricky topic is covered very well.
The book also covers running a git server, sensible workflows to tame distributed collaboration, useful/advanced git facilities (stashing, amending history, binary searches, subtree merging, client- and server-side hooks), and using git with other version control systems. In particular, there is good coverage of using git as a more advanced subversion client.
The final chapter is particularly useful: a great overview of git internals. This sounds relatively pointless when you've covered most git usage already. However, this is a great chapter - the author explains what's going on under the covers in such a way that you gain a much better insight into how all the high-level git operations work.