Pro Git
Ben Straub & Scott Chacon
2nd Edition (2014)
Git is an important part of a developer's tool set nowadays. It has become a quasi monopoly in the source control world, which is a rarity in tech. You probably already know some basic git commands, but this book is about learning the internals of git, and harnessing the full power of git. It's about unlocking the unknown unknowns. You can look up a git command anytime, but first you need to know whether git can in fact accomplish what you have in mind (which is almost always true). The best thing about this book is that it's totally free of charge, friendly to beginners, and you don't even need to read it whole to up your game. The first few chapters should do.