Malachi is the 11th least popular book of the Bible based on digital use and searches. This final book in the Old Testament is a dialog between God and the priests and people of Israel. Malachi is basically a back and forth about how many things displeased God about the Israelites and their worship and their lives. 

God (through Malachi) tells the people that they are not worshipping in the heart, they are not living faithfully. God says, "I am mercy. I don't change."  He calls for repentance and notes that He is patient.  Building the temple back didn't fix it. Observance of rules doesn't soften the heart. Beautiful but empty worship does not please God. God challenges them (and us) to change and see if He doesn't open a window from heaven to rain down blessings in return.