Some main points of the book I find interesting:
...There is a noticeable universalist tendency against Jewish intolerance, which misinterprets the teaching about the election of God's people.
...The purpose of the book is apparently religious and educational [3,10; 4.1 Him]. In the first part, God's omnipotence is shown, which can force a person to do His will. You will not escape God's sight either by suicide or in the underworld under the seabed! In the second part, God's is depicted the benevolence which forgives the repentant sinner, even if he were a heathen. It is therefore not surprising that the book was read on the great day of atonement every year.
The Jews saw it not only as a message about God's willingness to forgive, but also a symbol of their nation: Jonah is a representative of a nation that was sent by God into the world to prophesy to the Gentiles. When Israel did not fulfill his task, he was swallowed up by Babylon, not to be destroyed, but to give him an opportunity to repent [see Jer 51,34,44!]
In late Judaism, legends accumulated around J. For example, J. was considered the resurrected son of the widow of Sarepta, and J.'s reluctance to go to the Gentiles is interpreted in such a way that he wanted to prevent God from punishing the unrepentant Israel after He would see the repentance of even Gentiles.
The book of J. is the only writing of the OT, which explicitly encourages a mission among the Gentiles. But this is based on the assumption that Nineveh really refers to a pagan region. According to some interpreters, it is rather a cover name for Jerusalem.
(Biblický slovník, Adolf Novotný)
