This is the "Principal Message" about the book of Jonah taken from the NKJV Study Bible. I thought it summed it up nicely so I'm typing it up for you below.
Jonah
The Book of Jonah challenges GOD's people not to exalt themselves over others. The Lord, the great King, is free to bless, to be gracious, and to be patient with all the nations of the earth. More than that, he may show compassion even on the wicked. Indeed, His mercy extends even to animals (4:11).
Jonah's view of God was too restrictive. he believed that God was th ecreator of everything but that He was compassionate only toward the elect of Israel. Jonah believed that since God had chosen Israel from among the wicked nations, he had to show mercy to Israel, even if the people were rebellious. Jonah had failed to appreciate that the Lord may be equally forbearing with other nations as He was with Israel.
The Book of Jonah affirms God's freedom, sovereignty, and power. God is sovereighn because He is the Creator of everything (1:9). His power extends over all creation (the storm, the fish, the vine, the worm). God is free and He can never be bound by human misconceptions.
The self-righteous make the grave mistake of rejoicing only in their own deliverance (2:9) and in God's answers to prayer (4:6). They miss out by narrowing God's grace and mercy to themselves. Like Jonah, they cannot share in God's delight in saving the sailors and the city of Nineveh, including infants and even animals (4:11). They confess that God is Creator and King of the whole cosmos, but restrict His involvement to judgment, justice, and retribution. In this manner they do not see His acts of compassion, righteousness, and forbearance. The Lord's final proclamation to Jonah (4:10, 11) sums up the prophetic message of the book: God is free to bestow His mercy on anyone and anywhere He wills. His concern and mercy extend to all creation.
Jonah's story contains a strong warning to all godly people. The elect may miss the blessing of seeing God's grace extended outside their own sphere because of their imposition of limits on God. While Jonah was praying anxiously for his personal deliverance, the sailors had already been experiencing the love of God for three days. Likewise, the people of Nineveh who repented of their sins rejoined that the impending judgment had not come. Jonah, however was miserable. As we laugh at him, we may need to wince at ourselves. Jonah's silly sin is finally no laughing matter. We are condemned along with him if we share in his provincial folly.