The Book of Amos is set in a time when the people of Israel have reached a low point in their devotion to God - the people have become greedy and have stopped following and adhering to their values. The wealthy elite are becoming rich at the expense of others. Peasant farmers who once practiced subsistence farming are being forced to farm what is best for foreign trade, mostly wine and oil.
Yahweh speaks to Amos, a farmer and herder, and tells him to go to Samaria, the capital of the Northern kingdom. Through Amos, God tells the people that he is going to judge Israel for its sins, and it will be a foreign nation that will enact his judgment.
The people understand judgment as the coming of "the Day of the Lord." "The Day of the Lord" was widely celebrated and highly anticipated by the followers of God [Ed. Nowadays New Age Rainbow Christians]. However, Amos came to tell the people that "the Day of the Lord" was coming soon and that it meant divine judgment and justice for their own iniquity.
The central idea of the book of Amos, according to most scholars, is that God puts his people on the same level as the nations that surround it -- God expects the same sinlessness of them all. As it is with all nations that rise up against the kingdom of God, even Israel and Judah will not be exempt from the judgment of God because of their idolatry and unjust ways. The nation that represents Yahweh must be made pure of anything or anyone that profanes the name of God. God's name must be exalted.
Other major themes in the book of Amos include: social justice and concern for the disadvantaged; the idea that Israel's covenant with God did not exempt them from his position on sin; God is God of all nations; God is judge of all nations; God is God of moral righteousness; God made all people; God elected Israel and then redeemed Israel so that he would be known throughout the world; election by God means that those elected are responsible to live according to the purposes clearly outlined to them in the law; God will only destroy the unjust and a remnant will remain; and God is free to judge, redeem and act as savior to Israel.
Much of the prophecy of Amos is directed at the heartlessness of wealthy merchants who ignore the plight of the poor, the lack of justice for the righteous, and the emptiness of religious ritual apart from true faith. Amos is a classical prophet, concerned with the well-being of the people and the purity of the faith.