One who contributes from his wealth for the purpose of writing or publishing a sefer of a Tzaddik –
Behold, at any given time, and in any corner of the world, that a person studies from this sefer, the Tzaddik [on high] evokes merit on behalf of the donor through whom the sefer was disseminated.
—The Rebbe, Igrot Kodesh, vol 2
Written by the second Lubavitcher Rebbe, Shaarei Orah which is considered the "Aleph Bet of Chasidus," explains many basic concepts of Chabad Chasidus.