Nothing in the Constitution gave a President the authority to free slaves. The only thing Lincoln could use to make his actions legal was his authority as commander-in-chief in wartime. But that meant that he could only free the slaves in territory controlled by enemy forces.
It took not only legal shrewdness but much courage to do what Lincoln did. There was no big political support in the North for freeing slaves. In fact there was much opposition to the idea by Northerners who feared that such an action would stiffen Southern resistance and prolong a war that cost more lives than any other war in American history. More than ten times as many American died in the Civil War as in Vietnam.
Lincoln was out on a limb, both politically and legally. He could have been impeached. At a minimum, he expected to lose the next election and was surprised when he didn't. But today we see the spectacle of pygmies sniping at this giant.
As for the other slaves not covered by the Emancipation Proclamation, Lincoln worked behind the scenes to try to get slave-holding border states to emancipate them by state actions that would be beyond the jurisdiction of the U.S. Supreme Court.
Failing that, he prodded a reluctant Congress to end slavery by amending the Constitution. He did a lot of political maneuvering on a lot of fronts to accomplish his goal.
Professor Guelzo's book does more than give us some sense of realism about a major event in American history. Perhaps if we come to understand the complexities and constraints of Lincoln's turbulent times, we might not be so quick to seize opportunities to reduce other times -- including our own -- to cartoon-like simplicities that allow us to indulge in cheap self-righteousness when judging those who carry heavy responsibilities.