enigma vs closed book

enigma and closed book both are nouns.

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

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

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

Other synonyms of enigma include mystery, secret.

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