Correll and Johnston
This is the classic book of keys to the vascular plants of Texas. Although the taxonomy has not been updated, it's still the foundation of Texas botanist's reference library. These are new, fourth printings from 1996, the original was copyrighted 1979.
Hardcover, 1881 pages, all key, no illustrations, 6.5 x 9 (by 3 inches thick), 1979