Grimoire

A Grimoire is a text-based book of magic typically filled with instructions on how to create magical objects like talismans and amulets, how to perform magical spells, charms and divination, and how to summon or invoke supernatural entities such as angels, spirits, deities, and demons.

In many cases, the books themselves are imbued with magical powers due to their contents or as a security measure to prevent unwanted individuals from viewing their contents. This sometimes causes them to gain sentience to some degree.

General Information and Abilities
According to Ms Smith, a grimoire was originally just a normal book, but due to the magic that was imbued into it, it is now a kind of living creature or Liminal, although whether the book is truly "alive" is difficult to define. It is not known who created the grimoires, or where the magic used to make them came from.

A grimoire absorbs information from its surroundings through some unknown mechanism and then used that information to affect its environment through magic. Grimoires posses a variety of abilities, such as being able to absorb the consciousness of people reading the book and then trapping them in a fake magical world created by the Grimoire itself. This makes a grimoire potentially dangerous, but also one of the most mysterious Liminals.

Although grimoires are books, they also have some impact on their surroundings through the magic that flows through them. This is evident in Chapter 76, as Arbatel was aware that the four M.O.N girls were being sent to the Kayado public library to capture it, causing the grimoire to see them as a threat. Because of this, the grimoire changed its appearance to that of a normal manga to lure the M.O.N-team into a trap. Additionally, the grimoire was able to summon an ambulance each time, which would take the victims' bodies to a hospital for their own safety.

Magical Worlds
When a grimoire absorbs a person's consciousness, the person's body falls into a kind of coma. However, the consciousness is transported into a magical world created by the grimoire. The nature of this fake world depends both on the information the grimoire has absorbed from its environment and on it's own personality. In the case of Arbatel, this world corresponded to a high school, since for years the grimoire absorbed information from the romantic-comedy mangas set within schools which were stored near Arbatel in the Kayado Public Library.

Space within these fake worlds appears to be limited, as Kimihito noted in Chapter 75 through Papi's description that the world in the grimoire Arbatel is actually an endless loop where nothing exists outside of the school grounds. Any attempt to leave the school grounds will only result in the person arriving back at the building from which they started.

While most of the people within the magical world were real, some of them such the as the owner of the 7-Eleven store were just imitations created by Arbatel. Kimihito compared her behaviour to NPCs in video games, who don't have a will of their own and only say their given phrases.

Influence on the people trapped in the magical worlds
A grimoire can brainwash the people trapped within it, as seen in the case of Arbatel, as the trapped humans and Liminals actually thought they were students at the Grimoire High school. However, this brainwashing only worked on people who wore the enchanted seifukus that everyone automatically wore when they were drawn into the fake world. The brainwashing was broken when someone took off their school uniform, but the magic prevented anyone (except Papi) from doing this voluntarily, so the "students" wore their uniforms even while they slept.

Since Arbatel saw the four M.O.N-girls as a threat, they were not given the roles to students, but had to roam as the five Mysteries of Grimoire High School. However, Arbatel had no control over them and technically they could roam the school even during the day.

A grimoire can also directly affect the people trapped within it, as Arbatel made Tionishia grow to a height of ten meters.

Trivia

 * The ancient Egyptians thought magic could be performed by reading and performing specific incantations, and used collections of written spells in various forms; all collected together in bundles that will later become known as "books". Writing itself was also once considered a magic art.