From what I have learned, many historians consider the birth of the nation-state to be post-feudal Europe. Yet why would ancient civilizations of Sumer (where the populace was likely more homogenous than the modern USA), Greece, Egypt, Angkor-Wat, not be considered nation-states? All of these had people sharing some sort of commonality with a state being tied to them.
I would also like to ask for any book recommendations considering this particular topic, or any relevant sources. Anthropological, historical, and geographical books with a broad scope of human history would especially be appreciated, given my lack of knowledge about the topic.
This is not a perfect fit for your question, but while you wait for someone to answer, a couple months ago I was involved in a pretty interesting discussion of a related question: did the idea of nationalism really just appear after the French Revolution?
Although the category of analysis there is nationalism, as opposed to the nation state, it could help you think through how historians think about the development of the idea of the nation state in the 18th and 19th centuries.