The “Old Testament” and “New Testament” division in the Bible is so taken for granted that many Bibles actually start the page numbering over again, as if they were too separate books.
Yes it is true that one is written (mostly) in Hebrew and the other in Greek, but I don’t think that justifies the idea that they are separate books. If a difference in language is all it took, A hefty portion of Daniel would have to be torn out and published as a separate book.
Of course, there is a time gap between the “New Testament” and the earliest previous book. But in fact, there are two other time gaps.
So the Bible is made up of four “Testaments.”
Within the Hexeteuch, the first six books of the Bible, Genesis could be interpreted as first “Old Testament” and the following five books as First “New Testament.” Genesis, remember, is actually a compilation of ten books and were probably existent and recognized as Scripture before Moses was born.
Within the Hexeteuch, the Penteteuch, the first five books, can count as the first “Old Testament,” and Joshua as first “New Testament.” There is probably more backing in the rest of the Bible for this interpretation, since Jesus was a new Joshua leading the people to a new land and leaving Moses behind…
But let us move on.
Judges through Kings and Psalms & Wisdom
To use language from our “New Testament” the first group of books are the equivalent of the Gospels and Acts,while the second group are like the Epistles.
Some Prophets may also belong in this section. Not sure.
Note that Kings ends with the exile.
Chronicles through Esther and Prophets (but maybe some prophets go with previous “Testament”?)
Note that Chronicles is written from a perspective after the return from exile.
The Gospels and Acts and the Epistles.