Untypically for Macaulay it's an angry book- with something that feels a lot like self-disgust taking the place of her usual detached amusement. It came out just after the First World War which may be the explanation. There are few characters to like. After a while it turns into a sort of half-hearted murder mystery.
Potterism the word didn't catch on, Potterism the book is a relative failure (though very readable), but Potterism- the thing itself- flourishes over these defeats in unknowing self-delight.