Author
The author of this book may have been James the brother of Jesus. James was not initially a follower of Jesus, but Jesus appeared to him after his resurrection and no doubt at that time he became a believer. He soon became the leader of the Jerusalem church and remained a prominent identity until his martyrdom in AD 62. There is no indication as to when he may have written the book.
The letter itself makes no claim to apostolic authorship. In selecting the canonical books the early Christians were concerned with apostolic authorship and so the book of James was included because it was assumed to have been written by one of the apostles, or by James the brother of Jesus. Standing by itself, the work could have been penned as late as 90AD, as the letter reflects the social conditions in the Roman empire during the reign of Domitian. All we can say is that the writer is most probably a prominent Greek speaking Jewish Christian.
Purpose
The purpose of the letter is to encourage believers in the Christian life. They are called "the twelve tribes in the dispersion". This could mean that his readers are Jews, or it could just be a Jewish way to the describe the Christian church. The writer clearly sees great danger facing the church and so encourages his readers to endure their present troubles without aggression, to love their neighbours rather than hate, to aim at purity of religion rather than the externals (favouring the rich, favouring form, favouring worldly wisdom ) and to patiently look to the coming day of Christ's return rather than becoming impatient.
Structure
The structure of the epistle takes the form of a circular letter in the shape of a sermon, and consists of a series of exhortations on different themes. Each exhortation addresses a particular problem facing the church.
Theology
The theology of the letter is one of great debate. Martin Luther described it as "a rather straw-like epistle". He felt, as did quite a number of the reformers, that James gave too much weight to the perfection of faith through effort. Luther didn't want to exclude the letter from the scriptures, although others such as Karlstadt did. They felt that the letter undermined Paul's doctrine of justification by grace through faith.
It is probably true that James does not undermine Paul's theology. In fact, he simply doesn't address it. In approaching the issues of Christian morality, he only addresses the necessary doing of it. He is not concerned with the means by which a person gains or progresses in righteousness. If that was the issue he was tackling, then he would have to stand with Paul (and Jesus) on the ground of the theology of grace. Being righteous in the presence of God, and being that way in our day to day life (albeit imperfectly), is wholly a gift of grace appropriated through faith. James is dealing with practical matters facing the church. The doing of these things he regards as a fruit of faith. We can expect that these expressions of the law of love will be exhibited in the life of a believer. From the theological angle we would simply say that the morality outlined in the book of James serves as an instrument for the indwelling Spirit of Christ to fulfill his work of renewal in the people of God. As we read these practical examples of the law of love, we look to the indwelling Spirit to continue his work of renewal in our lives, and so shape that morality in us. So we seek to live out the law of love defined by James, trusting Christ to continue his work of grace in our lives.