The bottom line here is that there was much hack writing and speechifying in the years leading up to the Civil War arguing that cotton was king, that the slave states were being exploited by the free North and that an independent cotton confederacy would be a good thing. Colwell took up the challenge and wrote a response to some very dangerous ideas -- ideas which you shamelessly circulate even a century later, when we all know how harmful they were. How can one not commend him for that?
If you're still looking for a refutation of Kettell's pernicious book, try Samuel Powell's Notes on Southern Wealth and Northern Profits. Like Colwell's book, it's short. It's not an exhaustive treatment of the subject. Both books were written under the pressure of events to make a point that needed making. There is nothing wrong with that.