Beyond Susan Carter's summary of the genus in Eggli (2002), we have relied on a number of people, including John Lavranos, Mike Gilbert, Philippe Bisseret, Meena Singh, Gerhard Marx, and Susan Carter to help us sort out the Euphorbias. Additional resources include:
Carter, S., and A.R. Smith, 1988, Euphorbiaceae, in the Flora of Tropical East Africa: A.A. Balkema, The Netherlands
Eggli, U. (editor),
2002, Illustrated Handbook of Succulent
Plants: Dicotyledons: Springer-Verlag,
Singh, M., 1994, Succulent Euphorbiaceae of India: New Delhi, India.
Singh, M., 2002, Succulent plants of India: Noida, India.
White, A, Dyer, R.A., and Sloane, B.L., 1941, The Succulent Euphorbieae (southern Africa): Abbey Garden Press, Pasadena, California (2 volumes).
Our primary reference on this wonderful genus is Eggli (2001) with heavy reliance on:
Bally, P.R.O., 1961, The genus Monadenium: Benteli Publishers, Berne, Switzerland.
Pritchard, A., 2010, Monadenium:Cactus & Co., Italy.