The Rebbe directs the writer to study the kuntreisim Toras HaChassidus and Limud Toras HaChassidus for a clear explanation of the distinction between Mussar and Chassidus. He explains that the soul, though spiritual, becomes connected to physicality through the body, which can even obscure the G-dly soul. He encourages the regular study of Chitas (Chumash, Tehillim, and Tanya).
https://www.torahrecordings.com/rebbe/igroskodesh/015/009/5474