No single work adequately covers the role of Colorado in the Civil War. Most of the attention centers on the epic march of the Pike's Peakers to New Mexico, the Battle of Glorieta, and, of course, Sand Creek. However, there is much more to the story. Northerners and southerners alike flocked to the gold fields in the years preceding the Civil War, and enough Confederate sympathizers remained behind to cause trouble in the territory. In addition to internal strife and other home front issues, Coloradans fought a two front war, against Confederates in the Southwest, Kansas, and Missouri and against a variety of Indian bands threatening the overland trails.
Campaign and unit histories exploring the military role of Colorado's Union volunteers exist (some quite good, like Flint Whitlock's Distant Bugles, Distant Drums: The Union Response to the Confederate Invasion of New Mexico), and general works like Alvin Josephy's The Civil War in the American West and Ray Colton's The Civil War in the Western Territories: Arizona, Colorado, New Mexico, and Utah have some value, but nothing really puts it all together in a satisfying manner. Duane Smith's The Birth of Colorado: A Civil War Perspective serves as a standard reference in various bibliographies, but I found it disappointing in depth and breadth. Most recently, I came across a listing for a self-published work, The Pike's Peakers and the Rocky Mountain Rangers: A History of Colorado in the Civil War. However, the short description, which also doubles as an author bio, is so ill-conceived that I doubt adequate effort was made to present the manuscript in publishable form.
With a promising Nebraska study slated for the first of next year -- James Potter's Standing Firmly by the Flag: Nebraska Territory and the Civil War, 1861-1867--, perhaps someone will be inspired to do the same for Colorado.