The last book of the Old Testament is the prophetic book of Malachi. It was written around 425 BC. It's fascinating to consider the last message God sent to his people before a period of silence--a ~450 year silence that was broken by John the Baptist's pronouncement of the coming Messiah.
In terms of historical context, Malachi's prophecies occurred after Israel returned home from the Babylonian captivity. They rebuilt the city walls of Jerusalem and rededicated themselves to God. But after some time had passed, their faith wavered and doubts about God's love and presence festered. Through Malachi, God reminded them they were his chosen people and that he performed many mighty works in the past and gave many gracious promises of the future. The prophecies (some of them are listed below) will bring glory and judgement.
A prophet like Elijah will come to prepare the way (3:1) and encourage repentance (4:6)
God will come to the temple (3:1)
The Messiah (messenger of the covenant) will purify the sacrifices, the priests, and the nation (3:1-4)
He will witness against evildoers (sorcerers, adulterers, liars) (3:5)
God's name will be honored throughout the world (1:11)
There will be a judgement between those who serve God and those who don't (3:18)
Just like the people of Malachi's day, we too need persistent reminders of what God has done for us in the past and what he has promised us in the future. Perhaps you're facing a period of silence right now in which God's voice and direction are eerily absent. If so, consider reflecting on his historical works and future promises.