For people who wants to get ahead on the Independent Study (be independent, in others words), I would recommend that you cover two texts simultaneously:
Lentin Joseph’s Mastering ROS and
Gentle Intro to ROS by Jason M. O’Kane.
Mastering ROS covers the practical applications of ROS (essentially building the same differential-drive (two motorized wheel) robot), and Gentle Intro to ROS covers the technical details of ROS.
Going through the applications and occasionally looking at the technical details of ROS is, I think, the way to go.