Well it's not actually a sequel. Here is a collection of short stories and histories detailing the life of Alternia, a changeling queen impersonating the life of Princess Celestia. These are the stories of the ruler under her mask.
Reading the main story: Princess Celestia: The Changeling Queen is a requirement if you don't want to be spoiled or confused. I will try to provide background information when I can.
For readers of the main story, there are basically no spoilers if you've read past chapter 8. Enjoy!