I have become a collector of commentaries ever since my seminary days in the 1990s. As such, I am only going to recommend the best overall commentary for each book of the Bible on this site. (Before each minor prophet recommendation, let me first say that every preacher/teacher should have a copy of James Montgomery Boice's two volume set on the Minor Prophets on his/her bookshelf. That's the starting point, in my opinion, so I end in my recommendation of Malachi with Boice). That said, where is Allen Ross' "Malachi: Then and Now" on this page? If you are teaching on Malachi, make sure you consult Ross' commentary.