Author:

The prophets were preachers and usually not writers so, though the book bears his name and contains his words, it is not written by Jeremiah. The actual author is unknown, but Jeremiah stands at the time when prophecy changed from quotations passed down through oral tradition to written works that could be referenced. The reason for this change is the very events that are written about. With Israel and Judah both fallen and the people dispersed, it became very important for them to write down the words of their oral tradition to preserve their culture and identity. As the prophecies of Jeremiah span the time of the fall of Judah it is likely the first set of prophecies that existed in written form from the start of its use in the community.

Occasion:

Jeremiah preached both messages of impending doom and of hope in the midst of the disaster he said would come. The messages to repent and return to the ways of the Lord preceded the capture of Judah and Jerusalem by the Chaldeans/Babylonians. This message went largely unheard by his people. They appeared to think that God would save them from the danger even though they were not faithful to God's teachings.
After Jerusalem was captured, Jeremiah began to preach a message of hope. Even though the people had turned from God, God would not forsake them. Eventually, they would return to the land.
This writing also marks the time when the focus in corporate worship necessarily moved from the Temple in Jerusalem, with a focus on priestly sacrifices, to the local synagogues, with a focus on personal and corporate response to the words of God.

Place and People:

Jeremiah first preached in Jerusalem and then in Babylon during the exile. His first hearers were living in unstable times, but certain of God's protection. In spite of seeing Israel to the north fall, still believed that they were the chosen people of God and that God would not allow them to suffer a similar fate. Even as they believed this, they had largely turned from faithfully practicing the laws and teachings.
As the location shifts to Babylon in the exile, Jeremiah is preaching to a devastated people who are no longer sure of God's care and concern. He is also speaking to a people who are dispersed and in constant contact with people of other beliefs and world-views.