closed book vs enigma

closed book and enigma both are nouns.

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

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

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

Other synonyms of closed book include mystery, secret.

closed book (noun)	enigma (noun)
something that baffles understanding and cannot be explained	something that baffles understanding and cannot be explained
	a difficult problem