"Sefer Iggeret HaTeshuvah," which is a work by the Alter Rebbe, Rabbi Shneur Zalman of Liadi. It is the third part of the Tanya, the foundational text of Chabad Chassidus. Iggeret HaTeshuvah deals with the concept of Teshuvah (repentance), exploring its profound spiritual dimensions and the processes by which one can return to Hashem after straying from the path of Torah and mitzvot.