Conversation so far:
whats the last book of the old testament
The last book of the Old Testament in the Bible is Malachi.
who wrote the book Malachi
The book is commonly attributed to a unknown prophet by the name of Malachi, although its Hebrew meaning is simply My Messenger .
when was the book Malachi written
Most scholars assign Malachi to a position after Haggai and Zechariah, close to the time when Ezra and Nehemiah came to Jerusalem in 445 BC.

Question: why was the book written
Self-contained rewrite: why was the book Malachi written