The Old Testament containing 39 volumes starts from the Book of GENESIS that God created the universe.
God created the beautiful universe, hoping that human beings would have their happy times, obeying to what he told them to do. Unlike his hope, human beings sinned against God, which made God feel sad. So, he called Noah to destroy the universe by causing floods. He chosen Abraham to show that the Lord God would exist. Also, he commanded Moses to speak about the Ten Commandments, in order to rescue Israelites out of their slave lives in Egypt.
From that time on, he sent prophets out in the world and let them speak out against those who had been sinning against the God's wills. Israel that had turned back from God was left destroyed, in the long run, to be sent into exile out of the nation. Finally, God promised that he would send his only son Jesus out in the world, for those who would like to stay with God, so that God's will can be realized. The Old Testament ends in the Book of MALACHI in which God promised like that.