I would skip William's book. It is basically a reiteration of the 1988 book "Why the South Lost" by Richard , Beringer, Herman Hattaway, Archer Jones, Still. At 586 pages it is a more thorough treatment of the subject than William's.
But their thesis (and William's) has failed to convince many historians that internal divisions was a primary reason the South lost. Gary Gallagher and Jim McPherson haven both spoken against it. The North was divided , too, although less so than the South because they larger and more prosperous middle class. It was part of the reason the South less, but how much? Most of the historians who engage this argument use fugitive slaves as a substantial part of the equation (who knew the slaves didn't like the slaver owners? Wow). But what I think most people are interested in was the unity of the Southern white population.
These types of books have good anecdotal information, but I've never seen much useful data analysis. Its a good thing that the old nonsense of a unified South has been destroyed, but that happened in a 1925 book (and that still hasn't stopped the unified South perpetrators from twisting it to make it true).