Sefer Yoel (Joel) is the book of the prophecies of Yoel ben Petuel, and one of the books of the Neviim Acharonim (the Later Prophets). It is one of the Trei Asar (the Twelve), the twelve short books of the Neviim which, the Gemara says (Bava Batra 14b), are written together as one sefer. Yoel speaks to Yehudah (Judah) and Yerushalayim (Jerusalem).
The sefer opens with a plague of locusts that destroys the crops, until the mincha (meal offering) and the nesech (wine poured on the altar) stop in the house of Hashem. Yoel, a navi (prophet), calls on the elders, the kohanim (priests) and all the people to fast, gather in the house of Hashem and cry out to Him. He describes the day of Hashem as an army that nothing can stop, and he calls on the people to return to Hashem with all their heart, because Hashem is gracious and merciful.
Hashem takes pity on His people. He promises rain, grain, wine and oil, and He will make up for the years the locusts ate. After this He will pour out His ruach (spirit) on all people, and their sons and daughters will prophesy. Wonders will appear before the great day of Hashem, and everyone who calls on His name will escape.
In the last perek Hashem gathers the nations to Emek Yehoshafat (the valley of Jehoshaphat) and judges them for scattering His people, dividing His land and selling the people of Yehudah as slaves. The sefer ends with a spring flowing out of the house of Hashem and with Hashem dwelling in Tziyon (Zion).
Sefer Yoel has four perakim, and one haftarah comes from it. Pesukim 2:15–27, together with Hoshea 14:2–10, are the haftarah of Shabbat Shuva, read as the haftarah of Parshat Vayeilech.