I suggest reading this excerpt, from a book by Douglas A. Blackmon entitled Slavery by Another Name. I recall reading about his before, but this article (and book) appear quite in-depth. I think it's a very important component of US history, that is far too neglected.
I've been on a history-reading kick these most recent years – much more than the fiction I used to read. So I'm going to try to find this book if I can. The image at right is from the book. It's from the 1930s. Yes, those 1930s – not the 1830s.