In the introduction of the letter itself, Jacob introduces himself. He says that he is servant of God and of the Lord Jesus Christ (or Yeshua the Messiah, depending on your Bible version). He doesn't talk about how he is an apostle or disciple. He only seems to be concerned with service to the messiah and king - our service and his service, for that is the overall theme of his letter: How do we live and serve our Messiah?
Next Jacob tells us exactly who he is writing the letter to: he says that he is writing to the 12 tribes (of Israel) which are in the Diaspora, i.e. Jewish Believers in Yeshua the Messiah. This is important information. Many words and lessons of the Bible have been twisted and made a mockery because readers and pastors alike do not look at who the words were originally written to or what the word meant to those hearers. Jacob is writing the letter to JEWISH BELIEVERS IN YESHUA MESSIAH who live outside of the land of Israel.
The purpose of the book or letter from James is to encourage these believers in Yeshua to live godly, holy, set-apart lives IN SPITE OF and BECAUSE OF the trials and tribulations they will endure. He encourages them to realize that the trials and tribulations will mature them in their Messiah and that they will not be abandoned by their God.