I went to South Texas with my daughter Rachel. We visited the Coast, Lower Rio Grande Valley and then the Hill Country in March/April 2005 - about 10 days. We flew to and from Houston, and spent a couple of days working down the coast to Brownsville - for this we used "A Birder's Guide to the Texas Coast" by Harold Holt - one in a series published by the American Birding Association. We then birded the LRGV for about a week, where we used "A Birder's Guide to the Rio Grande Valley" by Lockwood,KcKinney, Paton and Zimmer - also from the ABA. They both were very helpful, especially the maps and site directions. We also used the Texas forum on Surfbirds for up to date info, which proved very useful, as it allowed us to catch up with some Mexican vagrants. I went to the Birdfair last week and came across a stand run by the Rio Grande Valley Partnership (
http://www.valleychamber.com): I haven't spent much time looking but it might be useful for motels etc.
As a Field Guide we used "Birds of North America", published by the National Geographic Society - fits into a pocket and pretty helpful. We also took "The North American bird guide" by David Sibley - wonderful illustrations, rather large for a pocket and the descriptions are a bit brief.
I hope this helps - if you want any further info please contact me by pm.
Rob Carr