Chofetz Chaim

The Sefer Chofetz Chaim is a book by Rabbi Yisrael Meir Kagan, who is also called "the Chofetz Chaim" after it. The book deals with the Jewish laws of speech.
The title of the Chafetz Chaim is taken from Psalms:
The book's subject is Hilchoth Shmirath HaLashon. Kagan provides copious sources from the Torah, Talmud, and Rishonim about the severity of Jewish law on tale-mongering and gossip. Lashon hara, literally "'the evil tongue", i.e., evil speech, is sometimes translated as "prohibitions of slander", but most commonly concerns the prohibitions of saying evil/bad/unpleasant things, whether or not they are true.
The book is divided into three parts:

The legal text is Mekor Chayim.Be'er Mayim Chayim, the footnotes and legal argument.
It is commonly printed together with the text Shemirath haLashon, an ethical treatise on the proper use of the faculty of speech.

The author

Yisrael Meir Kagan is commonly known as the Chafetz Chaim, the name of his book. He was born in Dzyatlava, Grodno Governorate, Russian Empire, on January 26, 1838. By 1869 his house became known as the Radin Yeshiva. Kagan published twenty-one books. His first work, Chafetz Chaim, is the first attempt to organize and clarify the laws regarding Lashon Hara. Other notable works include the Sefer Shmirat HaLashon, an ethical work on the importance of guarding one's tongue and the Mishnah Berurah which is a commentary on the "Orach Chayim", the first section of the Shulchan Aruch, and has been accepted among many Ashkenazi Jews as an authoritative source of Halacha.