Maiden Who Travels the Planet is a novella, that is not apart of the compilation.
This story, written by Benny Matsuyama, focuses on Aerith's journey in the Lifestream. Taking place immediately after her death and mid-way from the original game. I'm not quite sure if the book has been released or not, there is very little information about it. You can find more here, which also includes the English translation of the novel.