Documentation
If you want to know what a button or feature does, this is the best resource - Solid Angle’s documentation is excellent. This documentation is usually the first result when googling Arnold features.
Solid Angle - Arnold for Maya User Guide
Workshops/Videos
Alex Hartwin Film Lighting Workshop
Arvid Schneider Arnold tutorials
The videos below are an awesome resource - they are by a lighting artist at DICE and are super in-depth, focusing more on lighting theory than just technical stuff.
Unreal 4 Lighting Academy - Session 1.1 Simon Stålenhag scene
Unreal 4 Lighting Academy - Session 4.1 Dark Souls scene