One Fearful Yellow Eye by John D MacDonald (1978) Lippincott, Williams & Wilkins

A curiosity that feels like the wheels went flat, then reveals what you should have realized all along.  McGee is given a call for help from a woman in Chicago that he acted as matchmaker for a few years earlier.  A considerable sum has gone missing from her husband’s accounts.  The kids blame the new wife, the cook blames her wayward daughter.  McGee has other suspicions.