[Mexican Texans in the Union Army by Jerry D. Thompson (Texas Western Press, 1986) Softcover, 3 maps, photos, appendices, notes. Pages main/total: 41/87. ISBN: 0-87404-155-4]
Broadly speaking, the Hispanic contribution to the Civil War has been understudied, but historian Jerry Thompson has devoted a large segment of his research and writing career to the subject. At only 41 pages of main text, Mexican Texans in the Union Army [No. 78 in Texas Western Press's Southwestern Studies monograph series] is only a brief survey. The operations of irregular leaders such as Octavio Zapata and Cecelio Valerio are discussed, among others. Texas attorney Edmund J. Davis and prominent Unionist John L. Haynes attempted to form regular units [1st and later 2nd Texas Cavalry] from men recruited on both sides of the border. A brief operational history of these units is also included.
Mexican-American motivations to fight were complex. According to Thompson, they had more to do with anti-Texas [i.e. the state government and large landowners] feeling than any kind of loyalty to the U.S. or the Union cause in general. Inefficiency (to include communication difficulties) and a massive desertion rate, were probably factors in their ultimate obscurity, but external problems were equally significant. Due to questions about the units's legality, pay and equipment for these men were not forthcoming.
More detailed demographic information can be found in the appendices. Thompson also compiled a list of all known Mexican and Mexican-American individuals that served in the Union army. Each entry includes name, rank, unit, birth date, occupation, and date of death, if known or applicable. Packed with information, this slim volume is an excellent introductory-level study and valuable reference book [although I understand that the soldier list has been updated in the most recent edition of Thompson's Vaqueros in Blue and Gray].