We know that the Qu'ran was revealed to the Holy Prophet through Angel Gabriel. How the other books i.e.Psalms,Torah and Gospels were revealed to the respective prophets?
The Holy Qur'an says:
This verse says that the Almighty Allah communicates religious guidance to the Prophets (sws) through one of the following ways:
It is not possible for any mortal man that God should speak to him (face to face) except (that he does so) by revelation, or from behind a veil, or though a messenger sent and authorized by Him to make known His will. (42:51)
1. revelation (direct inspiration)
2- speech from behind a veil
3- through the angels
Muslims believe that the angel Gabrael was responsible for communicating with most of the Messengers, if not all. He has been a major source of transmission for all the divine books.