The Book of Jonah is read in the synagogue as part of the Yom Kippur afternoon service, to provide us with a striking example of the power of repentance. The Book of Jonah relates that the city of Nineveh was so evil that God had determined to destroy it. Before carrying out this judgment, however, He sent the (reluctant) prophet Jonah to warn the city of its impending destruction and give them one last chance to redeem themselves. The people of Nineveh listened to the prophet's warning and repented immediately, thereby achieving forgieveness.
May we all be written and sealed in the Book of Life!