We'll have to disagree on Mr. Zak's book. I think we can admire the personal courage and dedication of Confederate soldiers, but the principles that should animate the Republican party are found in Lincoln, Sumner and Stevens, not in the Democrats and Confederates of that age.
Not just Confederate soldiers. We can admire the personal courage and dedication of the Union soldiers too. After the war many soldiers of both sides recognized the valor of their opponents and came together for various reunions. For the most part, I think many Union veterans would have objected to shabby treatment and disrespect of the Confederate Battle Flag.
Slavery aside, there were some good parts of the Southern and Democrat philosophy of that age. I think the South had the correct interpretation of the Constitution concerning secession, states rights, protectionist tariffs, and the federal rather than national form of government.
Secession was the ultimate peaceful check and balance protection against an out of control central government or a de Tocqueville cabal of states that might be taking advantage of your state. I don't think states would have agreed to the Constitution in 1787 unless they had that kind of escape clause -- witness statements from Virginia, New York, and Rhode Island at the time concerning the right of resumption of government by the people.
I don't know a lot about Sumner and Stevens. My impression is that although Stevens treated blacks humanely, he was a vindictive Radical Republican who wanted to punish the South after the war. To Lincoln's credit, Lincoln differed with the Radicals over how the South should be treated after the war. I don't think that the punishment philosophy that Stevens imbodied was good for the country. Reconstruction added greatly to the bitterness felt by the South after the war.