Debra Holland is the creator and original author of the Montana Sky series of books, which became a Kindle World on Amazon. That means other authors can choose to write in the fictional world she created. Debra explains how Kindle Worlds work, and how they are different from each other in terms of how the creators work with the other authors.
She also discusses how the contracts and royalties work, how authors in her private group work together to cross-promote all the books together, things you need to understand about writing in a world that readers already have expectations about, and things she's learned along the way.
One more suggestion from Debra, even if you're not writing in a Kindle World - if you're going to write a series, start a book bible from the very beginning! It's difficult to begin one 18 books into the series. ;)
If you're interested in writing in a Kindle World, check out the list of them here.