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The demon books (妖魔本 youma-bon) refer to a category of rare books (稀覯本 kikou-bon) written by youkai. There are several variations of demon books: According to Marisa in the first chapter of Forbidden Scrollery, they consist of things like mere classics written by youkai, books which were written by youkai to address to humans, and grimoires for witches and magicians like herself. But the rarest set are the books that record the existence of youkai. Such books are explained by Marisa to be the last hope of the youkai whose existence has been denied, and whom are still waiting to recover from their sleep, which is why some of the books could be a threat to humans. Normally, they are not seen in the human village. However, Kosuzu has a collection of them, and there are some in Marisa's residence as well. Most of the books are written in a script that is unreadable to humans, with impossible-to-decipher characters. Kosuzu, though, is an exception, as she has the ability to decipher any book.
Ancient Tengu Language
The Ancient Tengu Language (古代天狗語 kodai tengu-go) is an obsolete language that was used by the tengu. According to Kosuzu, it appears that not only the humans, but even the tengu living now couldn't read it.
The Necronomicon is a famous fictional book that origins from the Cthulhu Mythos. In Touhou, a copy of the First Kanji Manuscript (第一漢字写本 dai-ichi kanji shahon) of the Necronomicon Manuscript (ネクロノミコン写本 nekuronomikon shahon) appeared in the first chapter of Forbidden Scrollery, located in Suzunaan. According to Kosuzu, the letters of the book are written in kanji, but the language it's written in and the author is unknown. Kosuzu mentioned that it was an expensive item when Marisa tried to "borrow" it as always.
- Wikipedia entry for the Necronomicon.