This summary is drawn from the Rebbe's letters to him (about 5717–5721, 1957–1961); I did not find a published biography.
Spreading Chassidus. The Rebbe wrote that the study of pnimiyus haTorah should be widespread among all Jews, not limited to a few (IK 5618); that both quality and quantity matter in spreading Chassidus (IK 6041); and, clarifying his own remarks about a weakening in the work, urged him to seize every opportunity (IK 5021).
From inspiration to action. Inspiration from a farbrengen must be translated into ongoing action (IK 4983), and Torah learning should keep growing, with careful planning ahead of Tishrei (IK 5735). True satisfaction comes from doing G-d's will with vitality and joy (IK 5643).
Questions. On the Zohar's teaching about teshuvah, the Rebbe explained that it stresses the obligation of teshuvah rather than guaranteeing its outcome (IK 5172). On his move to a new home, the Rebbe blessed it with added blessing (IK 7216).
Sources: the Rebbe's letters cited above.