http://git-scm.com/documentation
Documentation
Reference
Reference Manual
The official and comprehensive man pages that are included in the Git package itself.
Quick reference guides: Heroku Cheat Sheet (PDF) | Visual Git Cheat Sheet (SVG | PNG)
Book
Pro Git is written by Scott Chacon and published by Apress.
Print copies can be purchased from Amazon and PDF, mobi, and ePub versions are available for free.
All content under the Creative Commons Attribution Non Commercial Share Alike 3.0 license.
Scott Chacon
Pro Git