Return to the Road of Kings is the first book I bought after the core rulebook. I thought I had a good grasp of how the world worked before getting it.
Ymir blind me, was I in for a shock!
Simply put, this book is AMAZING. It is brimming with information on just about every single country that exists, and a few specific provinces here and there just for kicks. You get to understand not only how the country works, but how it thinks, feels, and is. The atmosphere of every single sector of the world is there, and it just lands in your face.
I constantly have my core rulebook in each and every one of my games, and my RoTK in my lap, turned at the page of the country my poor gamers are stumbling through.
I encourage every single player of Conan to get this book.