he phrase "ספר אגרת הקודש" refers to "Sefer Iggeret HaKodesh," which is the fourth part of the Tanya, authored by the Alter Rebbe, Rabbi Shneur Zalman of Liadi. Iggeret HaKodesh is a collection of letters written by the Alter Rebbe to his Chassidim, addressing various aspects of spiritual life, communal matters, and the importance of charity (tzedakah). These letters contain deep teachings on Jewish mystical concepts and practical guidance for living a life of Torah and Chassidus.

The letters in Iggeret HaKodesh cover a wide range of topics, including the importance of intention in mitzvot, the significance of giving tzedakah with joy, and the unity of the Jewish people through the observance of Hashem's commandments