The word Grimoire is a catch-all term that describes a compilation of alternatural, magical, spiritual, or Mythos lore. Because pure experimentation with Mythos sorcery is generally dangerous for humans, most sorcerers with an eye toward increasing their personal power seek out grimoires to add to their repertoire of capabilities. This information can be recorded in ink-on-paper books, but such knowledge can be found in almost any conceivable format: stone tablets, crystal shards, the whistle of wind through a cave, coded into fragments of birdsong, or within the genes of a particular human ethnicity. Once uncovered by a race, Mythos lore seems to be impossible to permanently eradicate.
Moreover, the act of personal genius and creativity that comprises composing a Grimoire infuses and animates the spirit/kami of the grimoire upon its completion. Collections or compilations of knowledge gleaned from third-hand experience tends to create willful books, capable of enticing others to read them or being capable of preventing their own destruction by conventional means; the book that refuses to be burned is a classic example of such powers. Mass-produced copies of the Necronomicon would have absolutely no animating spirit to them whatsoever. Only singular, original manuscripts-- those written in the unique vision of their markers, with first-hand experience on the course of their authors-- emerge as self-aware grimoires, capable of fully perceiving and acting on the universe.
Most of these spirits take the form of young human females between twelve and fifteen years old (and are called book-loli because of this tendency). These grimoires are valued above all others, because in addition to the knowledge written into them (which they can actively tutor interested parties), they possess the power to form a spiritual pact with a willing mortal being. In addition to gaining intuitive access to all of the grimoire's magical knowledge, pact-bound sorcerers have the ability to siphon substantial amounts of energy from the grimoire, using them as a sort of power battery. Sorcerers can also use grimoires as control/command interfaces for Deus Machina, the kaiju-sized mecha that starred prominently in the Arkham Event in the 1920s.
While grimoires are functionally ageless, they can be wounded or killed with sufficient force (though few would dare to do so, knowing what can be gained by seizing them in one piece). Sufficient damage can deface or destroy their contents, limiting their powers the same way selective amnesia might affect a mortal's ability to recall particular techniques or experiences.
While multiple grimoires exist within Suburban Senshi, only five possess known book-loli forms. Etheldreda, the Pnakotic Manuscripts, is known to be slumbering in the Void with her sorcerer-partner Master Therion. Al-Azif, the Necronomicon, is currently masterless after the death of her mortal love, and has hidden herself beyond mortal reach. Bixia Yuanjin, the Seven Cryptical Books of Hsan, took a young master from Mugen Academy that she met in Ten'ou House after being freed of her duties as Yaijinden's watchdog. The Tome of the Stilled Tongue also had a book-loli form that called herself Alria. Rue, who was held by Amber Royal, is the harbinger of The King in Yellow, and contains enough knowledge to raze the world, was spared because even though her contents were anathema, "her heart was still pure."