Why Joan?

Popular novel, first published in 1919. The story begins: "Young Joan Darcy leaned back luxuriously upon a cushion offered by the obsequious porter (servants were usually obsequious with Joan, though she was not at all beautiful and rather too shabby to promise much in the way of largesse), watching the world go by with a dreamy, detached, yet oddly observant gaze that missed no detail of the landscape through which she passed and registered it in her subconscious mind for future reference. It was a convenient receptacle, her subconscious mind--a sort of strongbox into which went many things valuable and valueless, to be brought forth when occasion required, quite intact."