directory vs reference book

directory and reference book both are nouns.

Word	Noun	Adjective	Verb	Adverb
directory	Yes	No	No	No
reference book	Yes	No	No	No

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

Other hypernyms of directory include book of facts, reference, reference work.

directory (noun)	reference book (noun)
an alphabetical list of names and addresses	a book to which you can refer for authoritative facts
(computer science) a listing of the files stored in memory (usually on a hard disk)