We rely on the work by Howard Scott Gentry as summarized in:
Gentry, H.S., 1998, Agaves of Contental North America: University of Arizona Press, Tucson, Arizona.
In addition, for more recent species we use the Monocots volume of the Encyclopedia of Succulent Plants.
Another useful reference is:
Irish, M., and G. Irish, 2000, Agaves, Yuccas, and Related Plants: Timber Press, Portland, Oregon.
Also see the Irish and Irish reference above.
Hochstatter, F., 2000-2004, Yucca I, II, III: Privately Published, Germany.