I've jokingly referred to this book as "the GM chapters of the 3E core" among friends, and I genuinely can't think of a better summary. The section on appropriate safety tools and best practices is worth it on its own, but Crucible also includes comprehensive advice on running different styles of game, playing social scenes, and more. The chapter on optional rules is a welcome addition on top.

Overall, I'd recommend this supplement to anyone planning to run a game. It's certainly served me well in that regard!