Located within Gensokyo's human village is Suzunaan, a bookstore that both sells and lends out books. Managed by Kosuzu Motoori, a huge bibliophile who loves collecting various types of reading materials, the store is known for having numerous books from the Outside World. Despite her seemingly innocent demeanor, Kosuzu has a unique ability to decipher all written languages—fueling her desire to secure even the most dangerous of books.

The deadliest books—called "youma books"—are written by youkai and embedded with their powers. Because of this, obtaining and stockpiling youma books in a small place such as Suzunaan poses an extreme risk that Kosuzu may yet encounter.