Joel is the Book of the Day of the Lord. Although other authors and books of the Bible mention the Day of the Lord, Joel stands as the great discourse on the subject. Joel used the occasion of a massive locust invasion to speak on a prophetic invasion that would leave the land with an even greater devestation.
The Hebrew, Greek, and English titles for the book are all taken from the proper name of the prophet who delivered the message of this book.
Joel, the son of Pethuel is otherwise unknown in the Bible. He was evidently from the southern kingdom of Judah, and possibly a prophet-priest like Jeremiah & Ezekiel. The time-frame for Joel, and his prophecy, is not entirely clear. The reigns of Joash, & Amaziah being the most likely for an early, pre-exilic date.