Version control
Before version control, I would code a bit and then when I became afraid of losing that code, I would make a backup of my directory. This is slow, inefficient, takes up a lot of disk space, and is very hard to share with others.
A Version Control System (VCS) does all this work for you (and more) and does so in a way that is fast, efficient, and takes up a minimum of disk space. One of the fastest and most efficient is Git, although there are others. This book will not spend a lot of time convincing you that Git is better than the others. First, the market has spoken, and Git prevails. Second, if you've purchased this book, you've already decided. And if you haven't already purchased this book, go do so. I'll wait here.