Old Testament vs Book of Ruth

Old Testament and Book of Ruth both are nouns.

Word	Noun	Adjective	Verb	Adverb
Old Testament	Yes	No	No	No
Book of Ruth	Yes	No	No	No

Old Testament (noun)	Book of Ruth (noun)
the collection of books comprising the sacred scripture of the Hebrews and recording their history as the chosen people; the first half of the Christian Bible	a book of the Old Testament that tells the story of Ruth who was not an Israelite but who married an Israelite and who stayed with her mother-in-law Naomi after her husband died