Good Book vs Word of God

Good Book and Word of God both are nouns.

Word	Noun	Adjective	Verb	Adverb
Good Book	Yes	No	No	No
Word of God	Yes	No	No	No

As nouns, Good Book and Word of God are synonyms defined as:

Good Book and Word of God: the sacred writings of the Christian religions

Other synonyms of Good Book include Bible, Book, Christian Bible, Holy Scripture, Holy Writ, Scripture, Word.

Good Book (noun)	Word of God (noun)
the sacred writings of the Christian religions	the sacred writings of the Christian religions
	the message of the Gospel of Christ
	a manifestation of the mind and will of God