His Book is selected to be read during Minchah, the Afternoon Service, of Yom Hakippurim, to show the power of Teshuvah, Repentance, and of HaShem's all-encompassing Midat HaRachamim, Attribute of Mercy.
His Book is the story of a Prophet who attempted to run away from his appointment to prophecy. One of the central mysteries of the Book is how this super-sophisticated Jew, considered to be worthy of Prophecy, could imagine that there was a place to run to, a place where he could hide, from HaShem? "And Yonah arose to flee to Tarshish." Surely he knew that the L-rd was there, in Tarshish, as well as everywhere else in His Creation!
Unless his love and appreciation of "Eretz Yisrael" were such as to make him think that Prophecy was possible only in and near the Holy Land (Yalkut Shimoni, "Shemot"/Exodus)
Yonah was a resident of Gat-Chefer in the area of the Tribe of Zevulon ("Melachim"/Kings II 14:25), and prophesied that Yeravam ben Yoash would restore the boundaries of Israel "from the entrance of Chamat until the Sea of the Aravah."
Chazal differ as to his origin; one opinion is that he was from Zevulon; another is that he was from the Tribe of Asher. They say that Yonah was the son of the widow of Zarephat brought back to life by Eliyahu ("Melachim" I, 17:17-24), and that he was the Prophet who, at Eliyahu's command, anointed Yehu and ordered him to destroy the House of Achav ("Melachim" II, 9:1-13, Yalkut Shimoni, Yonah, 550).
The Book of Yonah begins, "And the Word of HaShem came to Yonah ben Amitai, 'Arise and go to the great city, Nineveh (Capital City of Ashur),' and call out, that their evil (especially armed and violent robbery) has risen to the attention of G-d."
But Yonah hired a boat and fled. He did not want to see the repentance of the People of Nineveh avert HaShem's decree, while the People of Israel kept sinning. And he is chastised by CHAZAL, for he "took into consideration the interests of the son (Israel) and ignored the interests of the Father (HaShem)."
A great storm arose, and despite the great efforts of the sailors, they could not escape its ferocity. The sailors deduced that one aboard was responsible for the storm and drew lots, which fell upon Yonah. Yonah said, "Throw me into the Sea," (Yonah 1:12), again hard to understand how he, of all people, having already been brought back to life, could imagine that death was a place of refuge from HaShem!
Yonah was swallowed by a great fish (Yonah 2:1), and he prayed from the "belly of the beast" that HaShem save him. HaShem commanded the fish and he spat up Yonah onto the dry land. (Yonah 2:11)
HaShem commanded Yonah to go to the city, and call out, "In another forty days, Nineveh will be overturned" (Yonah 3:4 - this expression could be interpreted for good or for evil; if they would repent, the decree would be cancelled; but if not, Nineveh would be destroyed).
The King of Nineveh took Yonah's Prophecy very seriously, and commanded his People to fast and pray to HaShem with all their might! (Yonah 3:7-8; if only more Kings of Israel had reacted as had this non-Jewish King!).
As this had confirmed Yonah's worst fear, he complained to HaShem, " this was my thought that caused me to flee to Tarshish, for I knew that you are a Gracious and Merciful G-d, Full of Kindness, and Readiness to Forgive Evil." (Yonah 4:2)
"And now, take my life from me, for death is better than life for me." (Yonah 4:3)
HaShem then as-if toys with Yonah, to show the trivialities and frailties of human emotions. In the brutal heat of the day, He causes a gourd to grow and provide Yonah with shade. But then He causes it to be punctured and, overcome by heat, again Yonah asks HaShem to take his life.
And HaShem says to Yonah, teaching him the lesson of the gourd and of the flight to Tarshish, "You had 'mercy' on this gourd, that lasted but a day; Shall I not have mercy on Nineveh, a great city with a population, of greater than 120,000 human beings, who don't know the difference between their right and their left, and also much cattle." (Yonah 4:10-11)
HaShem is the All-Merciful G-d , Whose Mercies encompass all life, though they are infinitely far below Him, from the "egg of the ant," in the words of the RAMBAN, to the human being.