Lone Star Regiments in Gray (Eakin Press, 2002). The hardcover has been out of print for a few years now, but last time I looked it remained available in paperback format.
Texas supplied a whopping 53 cavalry regiments to the Confederate army, along with 24 infantry regiments, an artillery regiment, 4 artillery battalions, and 35 batteries. Divided into chapters by geographical region of service, all of these units are discussed by Wooster. He also elected to group many of them by brigade, instead of by the more common sequential presentation, a logical choice given that so many Texas regiments were afforded the unusual privilege of being able to serve together in the same brigade for the entire war. The capsule regimental histories are footnoted and provide basic organizational, leadership, and combat service information. Photos of prominent officers are present, as well as maps. While many of the regimental entries are quite lengthy, the book's artillery section is brief, with many batteries receiving only a single short paragraph. As so many Texas infantry and cavalry (mounted and dismounted) units, and nearly all the artillery, spent their entire Civil War careers in the Trans-Mississippi theater, the book is also a useful record of obscure formations. Wooster ends the book with an excellent bibliographical essay and index.