Creating layer masks is a critical skill to learn for making local adjustments to photographs. Experienced retouchers use layer masks to restrict sharpening effects, apply dodge and burn effects, etc. If you're still tentative about working with layer masks, Jay Kinghorn has a handful of tips to help you master them.
INSPIRE is a site from the Adobe Experience team. It's written in Felx and is without question one of the funkiest, quirkiest experiences for reading a tutorial. That said, they did a focus study of 75 new home users of Lightroom and listened in as they tried to work with their photos. This tutorial distills the top eight things that can help you get set up and working more quickly with Lightroom.