Here are a few links to additional resources that can help deepen your understanding of Github and serve as a reference for you as you learn it.
Courses & Online Books
Git Essential Training - Kevin Skoglund
Deep dive into Git with step-by-step exercises
Pro Git - Scott Chacon
Online version of Scott's authoritative book on Git.
Online Articles & Tutorials
Git Tutorials - Atlassian
Clearn, concise tutorials on Git.
Designing Github for Mac - Kyle Neath
Fantastic article about the design decisions made when creating the Mac Github Client. Offers insight into how the client works and perceived workflows.
Aha! Moments When Learning Git - Kalid Azad
Great article that focuses on the concepts important to fully understanding Git.