While the Claire Bible (Japanese: 異界黙示録 Kanji translation: Spirit World Book of Revelations Romaji: kurea baiburu) is mentioned as being knowledge imparted to man from the mazoku and the rest of the underworld, further along the Slayers NEXT series reveals to us that it is in fact the remaining power and memories of Aqualord Ragradia, created after the Kōma War.
A supposed infinite sea of wisdom, men have striven for eons to reproduce the knowledge of the Claire Bible. Manuscripts were produced, but mysteriously disappeared. The tales of one copy being burned was almost unbelievable to Lina Inverse until she met Xelloss one fine day, and everything started to make sense.
Rezo the Red Priest had a lithograph from the Claire Bible, with which he hoped to defeat Ruby Eye Shabranigdu. That lithograph was used by Copy Rezo to fuse himself with Zanaffar in his battle against the Slayers.
Lina did find one manuscript in the ruins of Kira's home, and from that copy, she learned to cast the Ragna Blade. Unfortunately, Kira's scribble drawings prevented Lina from learning how to control the spell. Xelloss was forced to destroy that copy to prevent Martina Zoana Mel Navratilova from using it.
In the Temple of Sand, the Slayers discover a complete copy of the Claire Bible, this one recorded in code on an infinite number of lithographs. Auntie Aqua, the guardian of the manuscript, led Lina to the lithograph that contained information about the Lord of Nightmares.
The true Claire Bible exists within a dimensional warp, also guarded by Auntie Aqua. The only known entrance point, on Dragon's Peak, was sealed during the fight between Lina and Chaos Dragon Garv. Xelloss has claimed that other entrance points exist, but has declined to reveal their locations. From this original Claire Bible, Lina learned how to control the Ragna Blade.
In the Novels
In the novels, Lina Inverse and company are only able to find manuscripts and it has been hinted by Lina that Xellos has been burning Claire Bible manuscripts throughout history. Unlike the anime version, Lina Inverse already had learned of the Ragna Blade spell prior to seeking out the Claire Bible but still wished to use the Claire Bible in order to learn the true nature of the Lord of Nightmares as well as find a method to defeat a high ranking demon.
Upon reaching the place where the Claire Bible is kept, Lina notes that she cannot see anything there. Milgazia responds that a Dragon's senses are far more advanced than a human's. Lina closes her eyes and senses the spot in the middle of the room where the Claire Bible resides and proceeds to link with it on a telepathic level to search for answers to her questions.
While learning of a method to defeat a high ranking demon one of Chaos Dragon Gaav's servants attacked forcing Milgazia and her to flee before she could obtain an answer.
While most of Slayers' runes are fully translatable, the animators seem to have thought they would only show briefly, because the ones in the Claire Bible amount to nothing, and are of a different runic alphabet (which seems to be anglo-saxon runes).