Over 5 years ago, shortly after GitHub initially launched, Chris pointed out on one of our earliest blog posts this Peepcode PDF on Git internals that I had just written:

peepcode-git pdf page 1 of 121

Well, today Pluralsight has agreed to open source the book under Creative Commons Attribution-ShareAlike license and the source is on GitHub. You can now download and read this book for free. Get it on its GitHub releases page and maybe learn a bit about how Git works under the covers.