This guide is very useful for creating depth in your campaigns, traps, skill challenges and a lot more. it's more than just supplemental material but it adds a lot to what's covered in DMG and other materials. Most of it perfected or refined by the one year since 4th editions release. As always great art.
I hate to be a fan boy and give it 5 starts all around, but I feel that this book is that good. I only wish that it were longer than 223 pages. It does a great job of giving tips and ideas to make you a better DM. It's not just loaded with a bunch of maps, monsters, and traps. Don't get me wrong, you get that too, but it really teaches you how to read your players, customize encounters to maximize their fun, and how to link those encounters together in a fun and meaningful campaign.