Well, I might be biased, but I think there's a place for both books on a bookshelf. :-)

The first book is aimed at getting your started down Git with little or no previous knowledge, the second book serves the same purpose assuming you have a basic grasp of VCS, but it serves a dual role as a reference.

Check out some of the excerpts that are available on the book's site to get an idea for what it looks like. The idea is that it gives you a quick reference when you're trying to remember how to do a particular task.