I found many sources helpful. Most are referenced or linked to throughout the text of the website. However, here is an additional list of a few:

Alexandrov, Valdimir E. Andrei Bely – the Major Symbolist Fiction. Cambridge: Harvard UP, 1985.

Gut, Taja, ed. Andrej Belyj – Symbolismus Ein Weg Anthroposophie. Dornach, Switzerland: Rudolf Steiner Verlag, 1997.

Janeček, Gerald. (1974) Anthroposophy in Kotik Letaev
Orbis Litterarum 29 (3) , 245–267 doi:10.1111/j.1600-0730.1973.tb01033.x

Kozlik, Frederic C. L’Influence De L’Anthroposophie Sur L’Oeuvre D’Andrei Bielyi. Frankfurt: R.G. Fischer Verlag, 1981.

Taschen, Benedikt. Wassily Kandinsky (1866-1944): the Jouney to Abstraction. Italy: Ulrike Becks-Malorny, 1994.

Thomas Beyer’s page on Glossolalia

These websites provided many of Kandinsky’s paintings:

The Tate Modern

ABC Gallery

Russian Avant-Garde Gallery