Funnily enough we do know (or at least can infer) for at least one of her books! theseatheseatheopensea's researches also turned up this short story, which appears (with very few changes) as one of the later chapters in the novel Sir Isumbras at the Ford. I was struck at the time by the method of turning a short story into a novel by exploring the background of one incident, rather than by simply expanding the plot into book form, and this discussion of her writing process really puts that in context.