The first six books of the Old Testament.
The first six books of the Hebrew Bible; there may be an underlying assumption that these belong together historically.
The first six books of the Bible considered as constituting one series
the first 6 books of the OT (Genesis - Joshua), from Greek "hexa-", six.
Hexateuch is a term used by historians and theologians to refer to the first six books of the Bible (the Torah or Pentateuch, and the book of Joshua).