The last time I checked, the Greek prefex “penta” means “five” (ie, pentagon). And, indeed, I have always understood that the Pentateuch refers to the first five books of the Old Testament (Genesis, Exodus, Leviticus, Numbers, and Deuteronomy).
My Catholic New American Bible (July 27, 1970, World Bible Publishers, burgundy cover with goldenrod text), duly authorized by the CCD and the USCCB, has a prefix with some handy information (list of Popes, list of miracles, etc). It also has a page listing the OT “Books of the Bible” arranged categorically (Pentateuch, Historic, Wisdom, and Prophetic).
Under “Pentateuch,” along with the five familiar books, it includes Joshua, Judges, and Ruth.
How does the Pentateuch become an Octateuch in my NAB?