Anonymous, possibly Jonah himself. Though there is some debate that Ezra (or some other post-exilic author) scribed this account. The evidence is simply too scant to take a decisive position; therefore, all suggestions are the best estimations of scholars.
God will have mercy and compassion on whomever He pleases.
Jonah is only mentioned one other time in the Old Testament (2 Kings 14:25), but from that reference we know that the events within the book of Jonah take place in the first half of the eighth century BC. The time was good for the northern kingdom of Israel. The Assyrian Empire had weakened because of its military conquests in the previous century. This gave Israel the chance to expand its lands, leading to a new prosperity among the people under Jeroboam II. Thus, there was not much perceived threat from the great empire that would actually destroy Israel by the end of the century.
The nation of Israel, therefore, had little to do with the Assyrians, along with one of its largest cities, Nineveh. And the distinction between the two peoples was sharp. The Israelites, after all, were the chosen people of God, and Nineveh was “the epitome of pagan wickedness.”
The primary purpose of the book of Jonah is threefold.
First, it displays God’s heart for the nations. Though Nineveh was a wicked and vile city, God sends Jonah to bring them a warning of judgment, so that they would have the chance to repent. Almost serving as a foreshadowing of the Great Commission, God reveals His great compassion for all people.
Second, the character of God is better known through this book. Though God’s action of being quick to show mercy to Nineveh, He exhibits a fundamental truth of His nature: that He does not delight in the destruction of the wicked and would much rather see them come to repentance (Ezek. 18:23).
Third, it provides for us a lens for viewing the depth of human depravity and frailty, even among God’s own people. In sharp contrast to God’s grace and mercy, Jonah stands as a testament to the bitterness and hardness of heart that the “religious” can build towards the pagans. Instead of rejoicing that God is loving and slow to anger, Jonah becomes furious because God does not punish Nineveh as Jonah thought it deserved.
Jonah is an interesting book when it comes to literature. Most of the prophetic books of the Old Testament are primarily composed of poetic oracles given from God; however, Jonah is primarily a narrative. Even the poetry found in Jonah is unique from the rest of the prophets, as it is a psalm instead of an oracle or decree. Still when compared to other narrative books in the Bible, Jonah is once more separate. The book of Jonah could be described as a satire. No, it does not poke fun at God or anything vulgar such as that, but rather the book presents the prophet Jonah is a satirical light. The entire book is filled to the brim with ironic and unexpected flairs. For instance, the prophet of God runs from God, while the pagan sailors on the boat cry out to God for mercy. The people of Nineveh react with surprising sobriety to Jonah’s message of judgment, while Jonah becomes bitter because God shows Himself to be merciful.
With the rise of textual criticism, modern scholars often deny the historicity of Jonah. Some choose to interpret Jonah’s adventures as being allegorical of Israel. Others believe that Jonah and his message to Nineveh are factual aside from the fantastical elements thrown in.
To be fair, it is easy to understand why secular scholars would write off Jonah’s historical reputation. This short minor prophet book is filled with supernatural works. In the middle of the Mediterranean Sea, Jonah is swallowed by a fish and lives in its stomach for three days. The most fearsome and pagan city on earth turned in repentance to God at the reluctant proclamation of Jonah. Finally, God makes a plant grows so fast that it makes Miracle Grow self-conscious. Yes, the book of Jonah is difficult to believe for the secular mind.
However, along with the majority of Christian history, I am convinced that these events are indeed historical. My reason for this conviction is found in the New Testament. Jesus refers three times to Jonah, each under the implication of Jonah being a historical figure (Matt. 12:38-42; 16:4; Luke 11:29-32). In Matthew 12:40, Jesus even briefly recounts Jonah’s three day stay in the fish’s belly. Since Jesus is God, I feel confident believing what He said to be historical fact.
Furthermore, it is difficult for me to understand how Christians can claim to believe the life of Christ but reject Jonah’s story as too farfetched. We believe that Christ was born of a virgin, never sinned, and was God incarnate. We believe that He died to pay the price for our sins, was resurrected, and ascended to heaven. We believe that Jesus is currently at the right hand of the Father, interceding on our behalf, and one day He will ride down from heaven on a white horse to rule on earth for all eternity. Every true believer will quickly say “amen!” to those statements because our salvation is dependent upon their validity, but the gospel is the most preposterous miracle of the Bible. The incredibility of Jesus outweighs all the other miracles in the Bible put together. If we can believe that God became a man to save us from death by dying for us, then surely we can believe that God is able to keep a man alive in the belly of a fish for three days
 Hill, Andrew & Walton, John H. A Survey of the Old Testament. P. 635