Sefer Malachi is the book of the prophecies of Malachi, a navi (prophet), 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 that, the Gemara says (Bava Batra 14b), are written together as one sefer.
The sefer gives no date. Malachi speaks after the return from the exile in Bavel (Babylon), when the second Beit HaMikdash (the Jewish Holy Temple) stands in Yerushalayim (Jerusalem), korbanot (offerings to Hashem) are brought on its mizbeach (altar) and a pechah (governor) rules the land (1:8). He speaks to Bnei Yisrael (the children of Israel) as a whole, to the kohanim (priests) and to the men of Yehudah (Judah).
Again and again the people question Hashem's words, asking in what way He loves them or in what way they have sinned, and Hashem answers. He loves Yaakov, as the ruin of Edom shows. He rebukes the kohanim who bring blind, lame and sick animals and despise His table, and He reminds them of His covenant with Levi. He condemns the men of Yehudah who marry women who serve idols and betray the wives of their youth.
Hashem will send His messenger and come suddenly to His heichal (sanctuary) to refine the sons of Levi and judge the wicked. He calls the people to return to Him and to bring the maaser (tithe). Those who fear Him are written in a sefer zikaron (book of remembrance). The sefer ends with a call to remember the Torah of Moshe and the promise that Hashem will send Eliyahu (Elijah) the navi before the great and awesome day of Hashem, to turn the hearts of fathers and children back to each other.
Sefer Malachi has three perakim, and two haftarot come from it. They are the haftarah of Parshat Toldot and the haftarah of Shabbat HaGadol, the Shabbat before Pesach.