Dungeon Master's Guide
Mike Mearls
This is a pretty good sourcebook, but really it’s not as important as the Player’s Handbook (or the Monster Manual), even if you are going to be DMing. There is a fair amount of general advice about designing an adventure, which I guess might be helpful if I were building from scratch rather than using a pre-written adventure like Curse of Strahd–but even then, I have to say a lot of it seems fairly intuitive. For me, the most useful parts are the magic item lists and the sections on creating or customizing creatures, spells, and items. I also love the random references that are halfway between crunch and fluff, like “Unusual Environments,” “Poisons,” and “Dungeon Dressing.”