<P>The personal ad posed a simple question: Are you happy? If not, consult Mr. Parker Pyne. The answer is a resounding no for a jealous wife who suspects her husband of infidelity . . . for a lonely widow driven to assume a new identity . . . for a distraught mother whose son has been kidnapped . . . and for the fiance of a strangely reclusive bride-to-be.</P><P>But what sort of detective would solicit in the personals? The sort who has a knack for investigating affairs of the heart, f or therein lie the darkest motives for murder. And the cases are proving most lucrative for the hopelessly romantic -- and suspicious -- Inspector Parker Pyne, a retired government employee who considers himself a "detective of the heart."</P><P>Parker Pyne Investigates features the first appearance of the characters of Ariadne Oliver and Miss Felicity Lemon, both of whom would go on to have working relationships with Hercule Poirot in later books.</P>