There is a lot to say about the book of Jonah. After all, people write good commentaries on the book even though it is only a few pages long. The book begins with Jonah fleeing from God because God wants him to go to Nineveh and warn the city of impending destruction. He eventually goes to Nineveh after a short sojourn in the belly of the great fish, and calls the people to repentance. He doesn't try too hard. (See 3:3-4) But it works; the people repent, much to the chagrin of Jonah. After the people of Nineveh repent and God relents from destroying the city, Jonah is displeased. He complains to God,
Is not this what I said while I was still in my own country? That is why I fled to Tarshish at the beginning; for I knew that you are a gracious God and merciful, slow to anger, and abounding in steadfast love, and ready to relent from punishing." (4:2)
It's tough when you are the last one to repent. It's a bit lonely I suppose. The Ninevites repent, and even God relents, that is, changes God's mind, which is a kind of repentance. But Jonah can't relent in his judgmentalism. And he is angry. So God tries to show him why God has relented. God brings a little bush up and then has it die. (4:6-8) Jonah is upset again, this time about the bush. And God says, in a way similar when Jonah was angry about Nineveh's escape from destruction, "Is it right for you to be angry about the bush?" (4:9, cf 4:4)
Idolatry is always hard to give up. That's why the 'idol' (see here) became an idol in the first place. Jonah has a set idea of God fuelled by his judgmentalism, anger, self-righteousness or similar. And not even God's enacted parable with the bush and explanation following, can change him. Well, actually, we don't know. The story ends without us knowing Jonah's response to God's explanation of God's mercy to the Ninevites. The book closes open-ended, functioning as an invitation to us to repent of hardness of heart, directed at others (and by implication ourselves).