The Git Book is the official tutorial about Git.
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Pro Available for free online
Pro Comprehensive and clearly written
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.