The main message of this book was that slavery was a system that was good not just for whites, but for blacks also.
Fitzhugh argued that slavery was the most just economic system possible. He argued that a "free" society was one that was inherently unfair to the bulk of people. He argued that such a society made the rich extremely rich, but allowed them to simply ignore the plight of everyone else. This was a very impersonal society in which no one cared about the poor and the weak.
By contrast, a slave society was very caring. Yes, some people did become very wealthy. However, unlike in a free society, these people took responsibility for the weak (the slaves). They cared for those people and made sure that they had a decent standard of living. Because of this, the slave system was much more humane than any system of free labor.
This defense of slavery was the main message of this book.