While examining the source materials used by the authors of modern studies of the Civil War along the Texas coast, one of the common works cited is James A. Irby's Backdoor at Bagdad: The Civil War on the Rio Grande (Texas Western Press, 1977. Wraps, 2 maps, notes, Pp. 61). This short pamphlet can reasonably be read in an hour and is a pretty good summary of the role the Rio Grande played in the Civil War. Irby introduces readers to the complexity of the blockade at the river's mouth and the tangled web of political/economic interplay at the border between Confederate, U.S., Imperial French, and Mexican interests.
Irby comes down somewhere in the middle of the debate over how important the Rio Grande trade was to the Confederate war effort. He blames the Confederate government's disorganized response for the general misuse of the economic opportunities the region offered. The Union war effort similarly failed to commit enough resources to choking off the cross border trade. On the other hand, he admits that the numbers needed to completely secure the border made such an effort unfeasible. While it remains a worthwhile summary, I would conclude that the expense incurred in obtaining a personal copy of this rather difficult to find monograph is not really worth it.