Book vs Word of God

Book and Word of God both are nouns.

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

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

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

Book (noun)	Word of God (noun)
the sacred writings of the Christian religions	the sacred writings of the Christian religions
the sacred writings of Islam revealed by God to the prophet Muhammad during his life at Mecca and Medina	the message of the Gospel of Christ
	a manifestation of the mind and will of God