field guide
Weniger, D. 1984. Cacti of Texas and neighboring states: a field guide. Univ. of Texas Press, Austin. 356 pp.
Glassberg, J. 1993. Butterflies through binoculars: A field guide to butterflies in the Boston-New York-Washington region. Oxford University Press: New York. 160 pp.
Powell, A.M. 1988. Trees and shrubs of Trans-Pecos Texas, including Big Bend and Guadalupe Mountains National Parks. Big Bend Natural History Association, Big Bend National Park, Texas. 536 pp.
Spackman, S., B. Jennings, J. Coles, C. Dawson, M. Minton, A. Kratz, C. Spurrier, and T. Skadelandl. 1996. Colorado rare plant field guide. Prepared for the Bureau of Land Management, the U.S. Forest Service, and the U.S. Fish and Wildlife Service by the