This summary is drawn from the Rebbe's letters to him, mostly from the 1940s, when the Rebbe headed Merkos L'Inyonei Chinuch and Kehot; I did not find a published biography. This page combines two tags, "Hovlin, Chaim" and "Hovlin."
Distributing Kehot books. Much of the correspondence concerns selling and distributing Chabad books and supporting communal needs (IK 122, 179, 241). Discounts may be given on printed Torah materials, but their content must never be compromised (IK 7784). He drew on Chanukah's theme of publicizing the miracle in discussing book sales (IK 122).
Communal work. The Rebbe urged proactive support for yeshivos and communal projects (IK 397, 357), and addressed communal logistics and fundraising challenges (IK 191).
Torah exchanges. The letters include detailed scholarly answers to his questions (IK 265, 287), why we say "Menachem Tzion" (IK 275), and how our approach to Torah observance shapes our spiritual state (IK 388). At a bar mitzvah, the Rebbe wrote that it marks both spiritual struggle and Divine help (IK 246).
Sources: the Rebbe's letters cited above.