Book of Esther vs Old Testament

Book of Esther and Old Testament both are nouns.

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

Book of Esther (noun)	Old Testament (noun)
an Old Testament book telling of a beautiful Jewess who became queen of Persia and saved her people from massacre	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