From the very first, this book is a contrarian examination of the Civil War, trying to make the point that the American Civil War was not, as it is sometimes called, the first modern war, but instead the last old-fashioned one.
While Griffith makes some interesting points, he also has the annoying habit of setting up straw men, and then setting them on fire, while using the light of the flames to read his next point.
For example, his attack on the Civil War use of cavalry is entirely based on his premise that there were no decisive uses of cavalry during the war [totally ignoring many tactical fights of significance], and that using cavalry as a raiding force is a waste of its potential. Of course, he also seemed to think that the armies of the Civil War should have used armored cuirassiers to make mounted charges. While it was possible to armor a man's torso in such a way that most bullets, even from a rifle, would bounce off, you could not similarly armor the HORSE. Also, some of the bullets would ricochet off the chest armor into the wearer's arms, also not a desired outcome.
Similarly, he assumes that the late-war reticence on both sides to charge headlong against fortifications was a bad thing, showing poor morale, when it could just as easily show that the men no longer trusted the tactic as having a likelihood of success. A man will dare much more when he's convinced he can win by being daring.
Still, he does make good points about how the fighting rarely utilized the technological advances in rifles and artillery, and that the armies on both sides were poorly-trained in actual combat skills, even late in the war.
Thus, the book is not great, but still worth reading.