Harriet is so irritating! Her whole personality and life history are just so much wishful thinking by the author. The constant self-analysis never discovers any real flaws and always serves to underline how very perfect and noble she is, in spite of much suffering and having to earn her own living - ugh! How can she write so totally unrealistic things?
As for Harriet/Peter relationship, that is also absolutely unbelievable. And every book that features Harriet makes Peter's character less plausible by this constant harping on how perfect he is, too. I admit other novels also imply that he's perfect, but they do it by hints and not by tiresome enumeration of talents and virtues.
All in all, it seems to me that Sayers should have stuck to the evidence-and-alibi-based mysteries, because she is no psychologist even at her best.