Why does this book's title refer to "the Second Revolution" even though there was only one US revolution?

The Civil War was a very revolutionary period in the United States.  By the end of the war, an entire race of people were made free whereas before the war they were slaves.  This changed the whole labor system in the South as the plantation owners now had no source of labor to work thousands of acres of cotton.  Before the war, the South had the majority of the nation's millionaires.  After the war, and for decades after, the South was the poorest region of the country. 
Seeing the Civil War as a new American Revolution also affects more than slavery and labor.  The North contracted to produce large amounts of uniforms, food, and weapons for its growing army.  After the war, these changes remained in place and made the United States into one of the fastest-growing economies in the world. 
The war also changed the relationship between individual citizens and the government.  The Union instituted an income tax for the first time as well as conscription.  These meant that the federal government could compel an individual citizen to give their time and money to the cause of fighting a war.  Many said that this was an intrusion of federal authority, but the income tax continues to exist and young men still register for the draft today, even though there has not been a draft in over forty years.  
The Civil War also changed the way that Americans see the country.  People are now more likely to say that they are a citizen of the United States instead of their own individual states; the Civil War cemented the idea that federal authority trumps state authority.  This is one of the important legacies of the Civil War, since no state has tried to secede since that time.