There is no complete field guide to Texas flora because it is such a large field. The following are useful guides.
Wildflowers of Texas
Geyata Ajilvsgi
1998, Shearer Publishing, $14.95 [Good photographic field guide]
Plants of the Rio Grande Delta
Alfred Richardson
1995, University of Texas Press, $24.95 [Keyed taxonomic guide with many photos together at the end of the book]
Wildflowers and Other Plants of Texas Beaches and Islands
Alfred Richardson
2002, University of Texas Press [Good photographic field guide]
Field Guide to the Broad-Leaved Herbaceous Plants of South Texas used by Livestock and Wildlife
James H. Everitt, D. Lynn Drawe, Robert I. Lonard
1999, Texas Tech University Press, $18.95 [Good photographic field guide]
Trees, Shrubs &Cacti of South Texas
James H. Everitt, D. Lynn Drawe, Robert I. Lonard
2002, Texas Tech University Press, $19.95 [Good photographic field guide]