Authorship and Origin:

According to the book itself, Zephaniah is descended from King Hezekiah. He lives and prophesied during the reign of King Josiah, which places his book around 621 B.C.. Zephaniah therefore was a contemporary with Habakkuk.

Overview and Significant sections

Zephaniah's message is clear, God is a God of judgement, and acts throughout history to judge as He will at the end of time, in the "Day of the Lord". Zephaniah sees judgement coming for Judah, the surrounding nations, and eventually all mankind.

Significant sections:

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