Pro Git is available at http://progit.org/ under Creative Commons (ie: free to read and modify). You can even clone the Git repository from which the online book is generated. After reading this review it should be clear that you should buy the book to support quality writing and the CC...
Pro Git is comprehensive from Git basics to collaboration and release management workflows to advanced Git wizardry. I have ordered copies for the office.
Scott Chacon writes concisely and clearly. Each chapter builds on the last in clear and sensible ways. After reading it I have a great deal more confidence in my ability to apply Git to my daily collaborative work.
I will be updating the workflows I had published on our internal wiki to reflect the best practices Scott describes.