I read A Defence of Virginia several years ago. From what I remember, Dabney attacked the abolitionists as undermining the authority of the Bible. He defended Southern slavery based on biblical teaching, though he criticized both the slave trade and the abuse of slaves.
I think Dabney’s book is important in providing insight into the theological debate over slavery in the 19th century. While many assume an abolitionist position today, Dabney challenges such thinking. He makes the case that abolitionism is untenable in light of the Bible permitting slavery in both the Old and New Testaments. Yet even if one accepts this position, I think you can still conclude that there were many laws and practices that should have conformed to biblical principles. Dabney also provided economic arguments for slavery, but I don’t recall the details.
While A Defence of Virginia is important, I would consider starting with Dabney’s other works. Though this may sound self-serving, I released an edited volume of four of Dabney’s essays for the purpose of introducing Dabney to modern readers – Dabney on Fire: A Theology of Parenting, Education, Feminism, and Government. I think my introduction and Dabney’s essays help make sense of his entire worldview, including his defense of slavery. The abolitionist mindset extended into other areas, and men began rejecting other authority structures like government and male headship in the home. Dabney sought to provide a consistent alternative to these attacks.