I haven't read much of Ngaio Marsh (or much of Wilkie Collins, and none of Alan Bradley). Usually if I don't like an author, I don't tend to read more. But there is a later Agatha Christie novel I don't care much for (I think I've read all her mystery novels)--Passenger to Frankfurt. It's one of her espionage thrillers, and (to my taste) not very thrilling. But the introduction to the novel (about how the novelist concocts her stories) is a revelation. Essentially, she makes everything up except the setting.