In Sefer Shemiras HaLashon, the Chofetz Chaim shows us the disastrous results of speaking lashon hara, and the innumerable rewards of positive speech. A masterpiece of mussar, it also includes chapters on the importance of Torah learning, of keeping peace and avoiding machlokes, judging favorably, and many more vital topics.
Sefer Shemiras HaLashon includes the full Hebrew text, a phase-by-phrase translation and elucidation, and explanatory notes to enhance our understanding.