Whilst reading The Outline of History during my lunch hour, I came across a rough progression of how the Bible came about. The first five books (collectively called the Pentateuch) had been around in verbal mythological form for a while. Then the Jews, migrating back from their captivity, took the Pentateuch, wrote it down, and then wrote the rest of the Old Testament. Around the beginning of the first millennium A.D., the Christians decided to add to the narrative again, producing the New Testament.
It seems to me that all this is like good movies and sequels. The Pentateuch is like Rocky: a great effort that should have stood by itself. The rest of the Old Testament is like Rockys II & III: they probably should have quit while they were ahead. The New Testament is like rockys IV & V: completely unnecessary and greatly diminished the impact of the original.
I'm not even going to get into the Mormons and any further possible Rocky sequels.