This is a Touhou LIGHT NOVEL doujinshi set somewhere after MOF.
The topic of dreams and reality often comes up in Touhou lore.
This book is the author's take on that topic. What reality? What dream? Whose dream?
And also that connecting that Maribel Hearn and Yakumo Yukai are hinted to have.
It got danmaku battles, yuri subplot and even a chaotic danmaku battle royal.