James D. G. Dunn is just about the best current New Testament scholar I've found in my all too brief and spotty survey of the literature. He has written a number of good books. Two that may bear on recent topics of this thread are The Partings of the Ways: between Christianity and Judaism and their significance for the character of Christianity (1991) and Unity and diversity in the New Testament: an inquiry into the character of earliest Christianity (1977).
Dunn holds that just as there was a spectrum of beliefs in Second Temple Judaism, there was also a spectrum of beliefs among the people, initially Judeans, who came to Christianity. He believes the thing that held Christians together was the confession "Jesus is Lord". This confession contains two elements, "Jesus", a real man who was really crucified, really died, and was really resurrected, and "Lord", the truth that this man has been invested with some sort of divine authority.
If we reconstructed the diversity of New Testament positions on a left-right scale, with right being conservative and left being for change, then the extreme left would be the gnostics, the moderate left would be Paul, the center would be Peter, the moderate right would be James, and the extreme right would be the ebionites. On the extreme left the gnostics went too far for most of the church to consider them to be Christians, because they denied that Jesus was a real man. They played up the "divine" business too much. On the right, the ebionites went too far in the other direction by regarding Jesus as just another man, with no special, unique exercise of divine authority. The gnostic and the ebionite positions were BOTH rejected by the majority of people who considered themselves to be Christians, but the positions of BOTH Paul and James were considered to be valid. It was through Peter that the church was held together in its early days.
It would seem that Luther read some of his aversion to the legalism of medieval Roman Catholicism back into Second Temple Judaism, and ignored the context of some of the things Paul wrote about "grace through faith". That may be why Luther thought that James should not have been included in the canon. But the early Christians themselves regarded James' opinions to be solidly within the bounds of their faith.
All for now.
Love,
Steve