I suspect you might really mean more generally International Relations, in which case reading intro IR textbooks or history about specific events is probably more informative if your goal is to understand "why do countries do the things they do and what are the 'rules'". Something like https://www.cfr.org/book/world might be a good place to start. It's relatively recent and I just gave it a skim and while it's certainly an American worldview it seems to stay pretty objective and I think it would be a worthwhile introduction.