A term used to denote a complete set of the documents, including all appendices, side letters, exhibits, schedules, etc. for a major commercial agreement – the documents usually required to be complete at “closing.”
Often large law firms used to send the complete set to bookbinders who would put hardback covers, often quite high quality on such collections of transaction documents, which caused them to bear a certain resemblance to religious books, e.g., bibles, hence the nick-name.