encyclopaedia vs reference book

encyclopaedia and reference book both are nouns.

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

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

encyclopaedia: a reference work (often in several volumes) containing articles on various topics (often arranged in alphabetical order) dealing with the entire range of human knowledge or with some particular specialty
reference book: a book to which you can refer for authoritative facts

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

encyclopaedia (noun)	reference book (noun)
a reference work (often in several volumes) containing articles on various topics (often arranged in alphabetical order) dealing with the entire range of human knowledge or with some particular specialty	a book to which you can refer for authoritative facts