The Hebrew Scriptures and its Greek translation (the Septuagint -- LXX), as well as books such as 1 Enoch, Jubilees, Sirach, etc., were the authoritative source for the "Jesus-follower" writers. Particularly as their foes - the Pharisees - accepted Tradition as being equal to the written word.
There is also no way that any of the NT writings is to be taken as literal history. None of it is a documentary in the sense that we understand. They were writing religious material, not documentary evidence. The Gospels and the Book of Acts are religious fictions, artfully crafted to manufacture religious outcomes and behaviours. The writings by Paul are simply his opinions -- not shared by all of his contemporaries.
Doug