The book is a comprehensive guide on everything you need to know to learn and master Git. It starts with the basics, but also moves into the deeper complexities of Git, not only explaining what commands do, but how.
A strong understanding of the inner mechanics of Git will help you use them to perform very powerful operations in keeping track of your code history, and developing features.
You cannot learn how to use git from this. You will quickly run into trouble and have no idea how to fix it. Try Git is a mere introduction to advertise Code School's Git Real course.
Many employers consider Stack Overflow reputation to be an authoritative or semi-authoritative source about the quality of a person's coding. Learning Git with Stack Overflow can help with learning Stack Overflow processes and best practices which could help land a programming job once you're ready to start answering question and really begin racking up enough points to get the prizes...
As a resource for learning Git, Stack Overflow can sometimes have a habit of presenting complex answers to simple questions and vice verse, and is no substitute for properly curated and differentiated tutorial content.
Right from the start, you'll be using git. You start small in an environment that has constant feedback and gradually progress to more complex concepts. You can see results instantly, giving great feedback.
Codeacademy doesn't overwhelm beginners with information. It gives bite-sized information, the bare minimum needed to finish a task and get to the next one. As the course progresses, it slowly fills in background information.
You can read about a very wide variety of topics, starting from basics and moving on to advanced features. Also, cheatsheets are available for quick reference.