It's actually a bit of both. The Chaosium book was used as a base, but the material was covered in more depth, since there wasn't space devoted to adventures.
It covers the history of the Dragon Isle, and the people who make it their home. There is discussion of the psychology of the Melniboneans, what their society looks like on all levels, including their drug use, slavery, and dreamcouches,
It also deals with the island itself. I've described the whole of the island, and filled it with places to explore and trouble to get into. The other islands that make of the Melnibonean Archipelago are also covered. Imrryr itself is covered in great depth as well.
Basically, I wanted to have Bright Shadows show Melnibone to be a place where an entire campaign could be run without ever leaving it.
