Authorship and Origin:

Very little is known about Joel except that he is the son of Pethuel. There is no date given for his message, although he mentions that Jerusalem is still standing, and the enemies he talks about are pre-Babylonian exile enemies.

Overview and Significant sections

Joel's message of judgement, is timeless, and affirms that God will act in history through judgement, and finally at the end of time. The book starts with describing a great cloud of locusts settling on Judah. Joel sees this as a judgement from God, calling for repentance. Joel further sees a similar event (judgement) coming at the end of time, which he calls 'The Day of the Lord'. Judgement however is always tempered by grace.

Significant sections:

For more detailed study:
Read it. (the entire book is available here.)
The Concise Matthew Henry Commentary on this book. study of this book.
the World Wide Study Bible has Dictionary, Commentary, Scripture and sermons available on this book.