Written by an expert in the field, "Pro Git" is a book with a misleading title, but in a good sense. It's useful not only to pro-level users of Git but also to newbies.
The first four chapters act as introduction/tutorial for a lower level of using Git. This part is especially useful for someone coming from different systems, like Subversion.
The rest of the book takes you on using Git at a higher level.
The book focuses on functionality first and on internals second. It exhibits excellent explanations and intuitive use cases. It is nicely illustrated, just enough to make the point.
Having an expert on anything to be also a good teacher on that field is rare. Scott Chacon has succeeded in being both.