Kramer, K.U. & P.S. Green (eds) The families and genera of vascular plants Vol. 1 Pteridophytes and gymnosperms. Springer-Verlag, 1990. (404 pp.) [Very up to date, somewhat technical, but providing an enormous amount of background information on the families and genera, with only occasional references to particular species. Well illustrated with line drawings. World-wide in scope.]
Butt, L.P. An introduction to the Zamiaceae in Australia. Palm & Cycad Societies of Australia, Brisbane, 1991. (55 pp.) [Illustrated, some in colour.]
Jones, D.L. Cycads of the world. Reed, 1993. (312 pp.) [The definitive guide, covering evolution, biology, conservation, and cultivation. Identification section includes a key to the world's genera, then genus and species descriptions with very high quality colour photos. A superb book.]