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Allow me to refer you to *Map Index to Topographic Quadrangles of the United States, 1882-1940* by Riley Moffat and published by Western Association of Map Libraries in 1985 (ISBN 0-939112-12-4). It is a graphic index with an alphabetical list of 394 15' and 30' maps covering any part of Texas. Obviously, it doesn't cover any USGS sheets published after 1940 or any of the new DMA sheets.
Best wishes,
Riley Moffat
BYU-Hawaii Library
Laie, Hawaii