The book is old so the writing style might be a bit difficult to understand now but if you can get past that it's an amazing book. Despite the serious tone of the subject, this was surprisingly fun and entertaining to read.
The book goes into brief history of feudalism in general, and focuses more on how feudalism works in Thailand's context. It covers a lot of the history that you probably won't find them in any mainstream Thai history book. I can see why it was banned. Having some background in Marxist theory helps makes this an easier read, but I don't think it's required.