The book of Malachi is the last book of the Old Testament, written around 430 BC. It addresses issues of the priests not properly conducting sacrifices and the people of Judah not tithing as required. Malachi warns that God will send a messenger to prepare the way for his coming. The book ends with a warning of God's coming judgment but also hope in Elijah preparing people's hearts and God's promise to bless the faithful. Overall, the book emphasizes that God sees humanity's disobedience and calls the people to repent and return to properly worshipping God.