Any student of the Civil War is aware of the fact that the Army of Northern Virginia was far more successful than the Army of Tennessee. The reasons for the split fortunes of the Confederacy between the East and the West are varied, and McMurry's book details them well.
The author argues that the Army of Northern Virginia had the proverbial deck stacked in its favor in many ways. It possessed better leadership, higher morale, a greater population from which to draw soldiers, military schools to train officers, greater access to railroads, more advantageous topography, and better military organization prior to the war in the form of organized, equipped, and trained state militias.
Of the Army of Tennessee, the author maintains that it suffered from poor leadership from the outset of the conflict, lack of trained officers, no military schools to train officers, and disadvantageous topography, among other issues.
Of these, McMurry devotes the greatest portion of the book to issues of leadership. The Army of Tennessee's leaders were quarrelsome amongst each other and with Confederate President Jefferson Davis. They were too often incompetent on the battlefield, and seemed unable to act in concert with each other. Part of the latter was due to the greater area in which they operated (seven states as opposed to only two in the East). The author, on no uncertain terms, argues that the enlisted ranks of both armies were skilled and brave troops, and that failure in the West was not the responsibility of the rank and file, but largely due to the generals who led them.
Finally, McMurry details how the Confederacy's two principle armies have been treated by historians. While the author acknowledges that history has been more generous to the Army of Northern Virginia and General Robert E. Lee, he also demonstrates that it is deserved. If history has not been as kind to the Army of Tennessee, it stands to reason, because nations tend to focus on military successes.
With that said, I think that the author might have addressed at least one issue in more detail. There is more to be said about the reasons for the second-rate leadership in the West than what McMurry details. It is true that General Robert E. Lee was a better manager of his Army and its leaders, and he did not tolerate failure or incompetence among his subordinates; however, when those subordinates failed, he sent them (or arranged for them to be sent) to the West, which undoubtedly exacerbated the leadership problems in that region. To my knowledge, failed leaders in the West were seldom sent to the East. Of course, there are exceptions, such as Beauregard in 1864, but by-and-large, the best leaders remained in the East. Of course the Army of Tennessee boasted some fine commanders, such as Cleburne and Forrest, but those were the exception rather than the usual.
Overall, if you want to know why the Confederacy's two main armies had such different experiences in the war, McMurry's book is the best place to start.