A likely story!
"... in accordance with the word of the LORD, the God of Israel, spoken through his servant Jonah son of Amittai, the prophet from Gath Hepher." 2 Kings 14.25
Thanks to his memorable encounter with a large fish and his notorious reluctance, Jonah must be the best known (and loved?) of the Minor Prophets. The verse above enables us to place him in history as a contemporary of Amos and therefore as operating during the 8th century, when the northern kingdom of Israel was enjoying a resurgence of military power and economic prosperity under kings Jehoash and his son Jereboam II.
Like the book of Ruth and unlike many of his fellow prophets, the book attributed to Jonah is an historical narrative, as opposed to topical declamations. As such, it features his mission as much as his message, from which we may assume that the two are linked. Of the various themes that may be identified, the principal ones are: the concern of God for nations beyond Israel and the reluctance of Israel (as represented by Jonah) to fulfil its role as light to all the nations.
Jonah exhorts us all to realise that God's intentions extend beyond what we may have assumed or even hoped; and that they are always and only good. In terms of the fish and its role, we may also discern confirmation to the effect that God is endlessly resourceful (and indeed witty) in making sure that his will is done, that his kingdom will come. Our sinfulness is no impediment - so we may as well just get on with being obedient!