''Chilling.'' Vogue ''As unusual as it is alluring.'' Elle ''Delightfully disturbing.'' Refinery 29 ''Very powerful.'' Sayaka Murata ''Disquieting.'' Paula Hawkins ''You will be obsessed.'' Leila Slimani The Woman in the Purple Skirt is being watched. Someone is following her, always perched just out of sight, monitoring which buses she takes; what she eats; whom she speaks to. But this invisible observer isn''t a stalker - it''s much more complicated than that.