Official Manual - written by Linus Torvalds and a few other authors
Git from the bottom up - Save this for the end and only if you want to become a Git expert. This book explains how Git stores its objects and dives deep into the Git internals. You don’t really need to know any of this to be a happy Git user
Pro Git - This is a free PDF book written by Scott Chacon, a Git expert who happens to be also very good at explaining Git concepts very clearly
Git Magic - A small and digestible overview of the main concepts