an analogous specialist writing in another discipline (say, a history of World War II, or a history of European painting, or a history of French jazz) would not have needed an equivalent to the 219 pages that make up Part 1: these can be loosely summarised as 'what is a folk song?'; and 'who collected the folk songs and how?'.

Sometimes they do. Books on the Crusades have a problem that they were mostly fought by people, on all sides, who had no label for what was going on - the modern idea of a "crusade" came along after it was all over. And books on the wars of the 20th century could certainly do with a recognition that both WW1 and WW2 started before they were declared and continued long after they officially finished, involving people who weren't recognized as combatants by any diplomatic protocol.