mystery vs closed book

mystery and closed book both are nouns.

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

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

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

Other synonyms of mystery include enigma, secret.

mystery (noun)	closed book (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