Malachi is the last of the Hebrew prophets until the time when Yahweh is going to judge the world, and he’s not particularly happy about it. Sure, the temple’s back in operation, but God’s people are back to their old tricks of mucking up the sacrifices and generally acting like jerks.
The prophet aptly named “My Messenger” spends his four brief chapters predicting the return of God’s ultimate messenger, the prophet Elijah, before the day when God condemns the wicked and rewards the righteous.
See, I am sending my messenger to prepare the way before me, and the Lord whom you seek will suddenly come to his temple. The messenger of the covenant in whom you delight—indeed, he is coming, says the Lord of hosts. But who can endure the day of his coming, and who can stand when he appears? (NRSV 3:1-2)
Malachi—which can also be translated “Mr. Crankypants”—scolds the people of Yahweh for violating their agreement, or covenant, with the notoriously judgmental deity, but their response to the arrival of God’s final messenger will keep the land from being struck with a punitive curse. Yet not all is bleak: Malachi says that in preparation for the Day of Judgment, God is writing a book that promises to be considerably longer than the twelve-scrolls-in-one Minor Prophets:
Then those who revered the Lord spoke with one another. The Lord took note and listened, and a book of remembrance was written before him of those who revered the Lord and thought on his name. (NRSV Malachi 3:16)
A few hundred years later, when the New Testament talks about God reading from the book of life on the Day of Judgment, it’s pretty much nicking that from Malachi.
And I saw the dead, great and small, standing before the throne, and books were opened. Also another book was opened, the book of life. And the dead were judged according to their works, as recorded in the books. (NRSV Revelation 20:12)
It’s a similar image in the Jewish liturgy on Yom Kippur, the Day of Atonement, except that in that case, it’s the living who are judged.
Everything we know about Malachi is what we have in the book known by his name, which, frankly, is not all that much. Commentators can, however, conclude that Malachi was indeed the author of the final book of the Hebrew scriptures based on his closing words, “Badeah badeah badeah badeah, that’s all, folks!”