Sefer Hoshea (Hosea) is the book of the prophecies of Hoshea ben Be'eri, and one of the books of the Neviim Acharonim (the Later Prophets). It is the first of the Trei Asar (the Twelve), the twelve short books of the Neviim that are written together as one sefer (the Gemara, Bava Batra 14b). (The Gemara, Bava Batra 15a, says that the Anshei Knesset HaGedolah (the Men of the Great Assembly) wrote down the Trei Asar.) Hoshea prophesies in the days of Uziyah (Uzziah), Yotam, Achaz and Chizkiyahu (Hezekiah), kings of Yehudah (Judah), and of Yeravam (Jeroboam) ben Yoash, king of Yisrael. He speaks mostly to Yisrael, the northern kingdom, which he often calls Efrayim, and to its capital, Shomron (Samaria, north of Jerusalem).
Hashem first tells Hoshea, a navi (prophet), to marry an unfaithful woman, Gomer bat Divlayim, as a sign that the land has been unfaithful to Hashem. Their children's names carry the message. Yizre'el (Jezreel, a valley in northern Israel) foretells punishment for the house of King Yehu, Lo Ruchamah (she is not pitied) says that Hashem will no longer have mercy, and Lo Ammi (not My people) says that the people are no longer His. Yet Hashem promises to turn the names to good, win Yisrael back like a wife and betroth her to Himself forever.
Most of the sefer is rebuke, because there is no truth, kindness or knowledge of Hashem in the land (of Israel). The kohanim (priests) mislead the people, kings are set up without Hashem and fall one after another, and the people serve the Baal and the calf of Shomron (avodah zarah). Efrayim runs to Mitzrayim (Egypt) and Ashur (Assyria) for help instead of to Hashem. Hashem says that He wants chesed (kindness) and knowledge of Him more than korbanot (offerings), and He warns of exile.
Even in His anger Hashem remembers His love for Yisrael from the time He called it out of Mitzrayim, and He will not destroy Efrayim completely. The sefer ends with a call to teshuvah (repentance), "Shuva Yisrael" (return, Yisrael), and with Hashem's promise to heal them and make them bloom.
Sefer Hoshea has fourteen perakim, and three haftarot come from it. They are the haftarot of Parshat Bamidbar and Parshat Vayeitzei, and the haftarah of Shabbat Shuva, read with a passage from Sefer Yoel as the haftarah of Parshat Vayeilech.