Kav ha-Yashar (lit. The Just Measure) authored by Rabbi Tzvi Hirsch Kaidanover, is one of the most popular works of musar literature of the last three hundred years. First published in 1705, it has appeared in over 80 editions, in nearly every country around the world. The work was famous for uplifting the spirits of Jewish communities in Europe after the Chmelnitzki Massacres of 1648-1649.
There is an English translation published by Metsudah.