One of the mysteries of the English language finally explained.
Definition of bankers' book in English:
bankers' book
(also banker's book)
noun
Originally: an account book belonging to a moneylender or money changer. Later: an account book or record showing the transactions of a bank; (now chiefly Law) such a book or record used as evidence in a court of law. Usually in plural Compare "bankbook".
Origin
Late 16th century; earliest use found in Samuel Daniel (?1563–1619), poet and historian. Partly from the genitive of banker + book.