From KanzakaDex

While the Claire Bible (Japanese: 異界黙示録 Romaji: ikai mokushiroku, but always referred as クレア・バイブル 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 strived 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.

Zelgadiss Graywords seeks the Claire bible to cure his chimeracism. Xelloss' quest and his clashes unavoidably, and the two are far from ever being allies, or even friendly enemies.

[edit] Trivia

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).