Generally it is easier to consider the whole bible to be the Word of God. But strictly, I guess not all of it is the Word of God. Depends how you define "Word of God". Obviously a copyright notice on the front cover didn't come from God. Or any words added by translators to aid the meaning.
If we limit that definition to only words which came out of God's mouth, so to speak, that includes the prophets, Jesus ( as He was God in the flesh), and the inspired teachings of the apostles etc who spoke by the Holy Spirit. But simple instructional or practical commands written by Paul for example I don't believe are God's Word. They are Paul's words. We may get inspiration from them, but I don't think we should say that God said this when it was only Paul speaking.