book of facts vs reference book

book of facts and reference book both are nouns.

Word	Noun	Adjective	Verb	Adverb
book of facts	Yes	No	No	No
reference book	Yes	No	No	No

As nouns, book of facts and reference book are synonyms defined as:

book of facts and reference book: a book to which you can refer for authoritative facts

Other synonyms of book of facts include reference, reference work.

book of facts (noun)	reference book (noun)
a book to which you can refer for authoritative facts	a book to which you can refer for authoritative facts