closed book vs mystery

closed book and mystery both are nouns.

Word	Noun	Adjective	Verb	Adverb
closed book	Yes	No	No	No
mystery	Yes	No	No	No

As nouns, closed book and mystery are synonyms defined as:

closed book and mystery: something that baffles understanding and cannot be explained

Other synonyms of closed book include enigma, secret.

closed book (noun)	mystery (noun)
something that baffles understanding and cannot be explained	something that baffles understanding and cannot be explained
	a story about a crime (usually murder) presented as a novel or play or movie