He is widely viewed as a prolific and original teacher; his work ranged from studies of the Torah, Talmud and works of mysticism to outreach and philosophy.
He made a translation into English of the Torah (The Five Books of Moses), published as The Living Torah. He wrote a number of highly popular and influential introductory pamphlets to many aspects of Jewish theology which were published by the National Council of Synagogue Youth, the official youth group of the Union of Orthodox Congregations (also known as the Orthodox Union, or OU.)
Having passed away at 48, Kaplan's works continue to attract a wide readership. He has been praised by Rabbi Pinchas Stolper, his original sponsor, for never fearing to speak his mind. He saw a world of harmony where others saw otherwise. He put forward creative and original ideas and hypotheses, all the time anchoring them in the Torah and Talmud of Jewry and their ancient rabbis.
Rabbi Kaplan translated from Ladino (Judeo-Spanish) into English a 45 volume rabbinic work called Me'am Loez. He translated into English and wrote notes and introductions to classic works on Jewish mysticism, such as the Sefer Yetzirah (Book of Creation) and the Bahir (Book of Illumination). He also wrote on Jewish forms of meditation.
He wrote and translated works related to Hasidism (Hasidic Judaims) and the teachings of Rabbi Nachman of Breslov.
His works are read and studied by both novices and the newly religious.
Translations have subsequently been made into other languages such as Russian and Modern Hebrew.