>* Imagined Communities - the notion of a "nation" is only 300 years old and has no objective basis, only the fact that a group of people agree that it is a thing.
The Penguin History of Europe series is great for this, especially The Pursuit Of Glory, which details the time when states switched from being based on their king, to states being based on a 'unified set of people', i.e., an imagined community.
There's also the amazing Vanished Kingdoms: The History of Half-Forgotten Europe, which is a bunch of essays, one on a forgotten European kingdom that ceased to exist, and no-one claims it as their heritage. It shows you how easily your identity of a citizen of a state can get lost and forgotten - your great-grandparents may have seen themselves as Etrurians, but that state is gone and now you think of yourself as an Italian, but nothing much changed about your family