The genus Sansevieria is particularly problematic given the strong contrast, or difference of opinion, between collectors and taxonomists. Add to that the fact that the genus is extremely variable and in dire need of taxonomic revision, and the potential for confusion is large. Until a complete revision of the genus is completed, that confusion will persist and permeates our species listing. Despite its problems, the single best reference on Sansevieria is:
Chahinian, B.J., 2005, The Splendid Sansevieria: privately published.
A good but deeply flawed review of the genus in East Africa is:
Mwachala, G., and P.K. Mbugua, 2007, Flora of Tropical East Africa, Dracaenaceae: Royal Botanical Gardens, Kew, UK.
A good reference on cultivars is:
Chahinian, B.J., 1986, The Sansevieria trifasciata varieties: Trans Terra Publications, Reseda, CA.
Finally, for what it is worth, the following is downloadable:
Stover, H., 1983,The Sansevieria Book: Endangered Species Press, Tustin, CA.