James is a crucial book because without it we might not be clear that real faith issues in works. James did not mean that our works justify us, he meant that works confirm the existence of justifying faith.
As one teacher said, the equation is not,
Faith + Works = Salvation.
it is instead,
Faith = Salvation + Works
In other words, real faith issues in two things (1) salvation, (2) works. This is what James was trying to convey.
The idea of justification by faith already was widespread when James wrote his letter, otherwise he would not have addressed the idea. But he did not challenge it, he qualified it. He challenged the false idea that all you had to do was say you had faith and you were saved. We know that to be true now because we know "nominal" Christians claim to believe but don't have works. They lack genuine saving faith that results in regeneration.