Oxidation state is both the simplest and the hardest concepts of chemistry. To be honest, sometimes (maybe more often than sometimes), I get confused too. In fact, you can even find slightly different definitions of oxidation state depending on what you are trying to reach in the end. To be honest, I found this paper a little bit confusing in the beginning. It's not the author's fault of course. Once again, oxidation state is not really straightforward. Overall, I think it is a nice paper to read and a good summary of interesting examples.
BONUS : It's open access.