We can learn a lot by considering the chronological order of the NT books:
http://www.biblestudytools.com/resou...testament.html
James was the oldest book of the New Testament. James was not up to date with the latest revelation from God about salvation by grace alone. Paul had yet to write his epistles. As one of the earliest books in the Bible, James teaches faith+works. For this reason the book of James is more the "expired" Word of God, than the inspired Word of God.
Consider the person of James himself. James as Jesus's half-brother was basically a second-class apostle who was not an authority in the church. See John 7:5. For this reason Peter refers to Paul's letters, not James.
James was really written to the Jews who had been dispersed amidst the Roman Empire during a time of intense persecution. Instructions for Gentiles were different (Acts 21).