The first Bible was likely a record of creation handed down from Noah to his sons. Then, Abraham's family kept records. From this, Moses was directed by God to write what is now Genesis. At the same time, he wrote down his own story, including the Law. This became the Scriptures up until the time of David.
Some people of this time period, and quite possibly Moses, knew of the story of Job. It is not known when that was written down, though.