The first chapter of the DMG is good for setting up expectations and guidelines for what your role is. As a supplement, I also like the Dungeon World GM section. It's been a bit since I've read it, but it had some more 'intro to GMing' styled information.

If they're looking for more materials like "How do i build an encounter" or "what makes a fun exploration section" or other mechanics focused things, I got... nothing. Sorry.