The Spread Shot of Kosuzu Motoori from the world with an ultra-forbidden scroll (a Touhou LW original). She commands one of the hundred haunted housewares in this attack. Demon books are a kind of magic item and basically include all books that exist outside the scope of human comprehension. For example, books, like grimoires, that facilitate magic, books written in a youkai language, books that are somehow imbued with magical power, books that act as a prison for youkai, and books that are themselves youkai.

The Focus Shot of Kosuzu Motoori from the world with an ultra-forbidden scroll (a Touhou LW original). She commands one of the hundred haunted housewares in this attack. Kosuzu's ability to read any book, including demon books, can be quite handy depending on how it is used. If she lived in the outside world, she could have used it to read every language, including programming languages, and perhaps even advanced mathematical formulas, becoming a child prodigy. Of course, this would never happen, since she only has her ability because she lives in Gensokyo. How many more children in the Human Village have abilities?

A Spell Card of Kosuzu Motoori from the world with an ultra-forbidden scroll (a Touhou LW original). She commands two of the hundred haunted housewares in this attack. Old tools become tsukumogami, a type of youkai, when they are abandoned for an exceptionally long time without being broken. This is because all tools function thanks to the magic of their user, and the more they are used, the more magical power builds up inside them. If you wish to cast a tool aside and stop using it, you must break it and perform a proper memorial service. Otherwise, it will return as a tsukumogami bearing a grudge that it will never forget. Due to the magical energy that flows through them, they cannot help but obsess over their former owner.

A Spell Card of Kosuzu Motoori from the world with an ultra-forbidden scroll (a Touhou LW original). She commands three of the hundred haunted housewares in this attack. It is often assumed that the Night Parade is a procession of oni, but the most gifted and treasured artworks of the parade depict it as a procession of tsukumogami. Perhaps the main reason for this is that tools imbued with oni magic are known to be especially potent magic items.

The Last Word of Kosuzu Motoori from the world with an ultra-forbidden scroll (a Touhou LW original). She attacks with her Night Parade Scroll's tsukumogami. When Kosuzu got spirited away, she might have caught a glimpse of the outside world and seen the humans living there. If she did, that would definitely cause her to have some concerns about the current state of Gensokyo. And with the Night Parade Scroll and her ability to read demon books, she has the power to act on them. It's possible that Kosuzu felt a sense of kinship with the old youkai trapped within the scroll. They had been ignored for such a long time, and while so many gifted and talented people surround Kosuzu, they always leave her out of the loop. Perhaps the youkai saw that they had this in common, too.