Most books in the Middle Ages were written by the:

Answer: by monks or university professors (Roger Bacon, St. Thomas, William of Ockham, Duns Scotus etc etc).

Explanation: in fact many medieval books were copied by monks. Monasteries were frequently centers of copying but also translating. Monks were usually (not always because not all the religious orders were focused upon education) quite educated in various languages, translation. Books were usually translated from Arabic, Greek or Hebrew language to Latin.