1) The sacred scriptures of Christianity and (without the Christian New Testament) of Judaism. Until the Reformation the text generally used in the West was that of the *Vulgate. At a time when all texts were handwritten, Bibles were usually to be found in two or three or more large folio volumes. Monasteries would have one as a treasured possession. The earliest complete Bible, i.e. both Old and New Testaments, surviving in the West is the 7c Codex Amiatinus.
2) This word, with a lower-case initial 'b', was given to the long moralising works common in 14c literature. This usage, orig. French, was familiar in England, where French literature exerted much influence.
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.