breviary vs prayer book

breviary and prayer book both are nouns.

Word	Noun	Adjective	Verb	Adverb
breviary	Yes	No	No	No
prayer book	Yes	No	No	No

As nouns, prayer book is a hypernym of breviary; that is, prayer book is a word with a broader meaning than breviary:

breviary: (Roman Catholic Church) a book of prayers to be recited daily by certain priests and members of religious orders
prayer book: a book containing prayers

Other hypernyms of breviary include prayerbook.

breviary (noun)	prayer book (noun)
(Roman Catholic Church) a book of prayers to be recited daily by certain priests and members of religious orders	a book containing prayers