“Secession Debated” is worthwhile, as is “Showdown in VA,” and Crofts’s book is required reading in grad school on the topic, but I would avoid Dew’s book. It is an open polemic against “neo-confederates” and his “evidence” is often cherry picked to fit his agenda, namely that only a defense of slavery drove secession during Dec-Jan 60 and 61. For example, several of these “apostles” visited Delaware in 1861 and catered their speeches to the views of that State, namely they promoted the economic motivation for joining with the Confederacy over pro-slavery sentiment. Of course, DE did not secede, but it probably would have if MD had not been coerced to stay in the Union by the Lincoln administration.
Certainly, the public declarations for secession expressly stated that Northern hostility to slavery caused the separation, but there were many Southerners uncomfortable with this position, and if the protection of slavery was the only issue, why did the South reject Corwin’s proposed 13th Amendment, which would have prohibited the central government from interfering with slavery in States where it already existed? It would have left the issue open in the territory, but according to many Southerners, the SCOTUS had already decided that problem with Dred Scott. We can argue how that would have been enforced, but many Southern leaders openly declared after 1857 that the issue was dead.
Was racism involved in secession in 1860-61. Yes. Was racism involved in Republican promotion of “Free Soil, Free Labor, Free Men” from 1854 forward. Yes. Were 19th century Americans racist (including Honest Abe and most Notherners)? Yes. Did Northern theological instruction contribute to a Biblical defense of slavery in the South? Yes. When people start placing racism front and center as “the” issue in 1860 and 1861 it distorts the picture into a “good vs. bad” moral crusade won by the North, which was simply not the case. The 1850s and 1860s were more complex and should be treated as such. Many Southerns believed that abolition would bring the same results as Haiti and Santo Domingo to the South, meaning the extermination of white Southerners. They had historical examples and thought that would be the natural outcome of such a move.
Here are a couple of good older books that I think do a better job on secession than Freehling (an open anti-Southern historian), Dew, and Crofts:
E. Merton Coulter: “The Confederate States of America.”
Richard Taylor (son of Zachary Taylor): http://books.google.com/books?id=C3QFAAAAQAAJ&printsec=frontcover&dq=richard+taylor+destruction+and+reconstruction&hl=en&sa=X&ei=a8HMULCUFoeC8AS40oDACQ&ved=0CDEQ6AEwAA
B.H. Wise: http://books.google.com/books?id=yswzAAAAIAAJ&printsec=frontcover&dq=life+of+henry+a+wise+of+va&hl=en&sa=X&ei=LMTMUOmQCo2y9gTk2oG4CA&ved=0CDEQ6AEwAA#v=onepage&q=life%20of%20henry%20a%20wise%20of%20va&f=false
Dwight Dumond, “The Secession Movement 1860-1861,” and “Southern Editorials on Secession.” Should be able to find both in a good academic library.