Those in the Japan camp are waaaaaaaaaaaaaaaay too focused on the technicalities. You'd be hard pressed to find any book that introduces "Joseon was a tributary of China" or "Joseon was a sovereign country/nation/state." No, it'd more like "Joseon was a country in Korea" the end. So that whole google book page hit is irrelevant. And Bill Price is right, the English terms are a sort of a cop out. I mean surely you wouldn't call the British Empire a tributary of China prior to the Opium Wars. Despite this denomination, Joseon was and acted as a independent sovereign nation. This excerpt explains more about the relationship and how such terms may be confusing.