This Sweet Sickness is a classic Highsmith book. I've read all of her novels, and this one has always held the place of my personal favourite, even though it is less well known than the Ripley novels, or Strangers on a Train.
David Kelsey is a typical Highsmith protagonist: self-obsessed, arrogant and opinionated, with a good income and a comfortable middle-class lifestyle. He has become infatuated with his former fiancee Annabelle to the extent where it dominates all his thoughts. He has become convinced that it is only a matter of time before he persuades her to come and live with him and resume the life they briefly had together - in fact the reverse is the case.
At weekends he has established an alter ago, William Neumeister, who is the perfect partner of his adoring wife - except it is all pretend! As the events in the book unfold (without spoiling it by going into the details) Kelsey is forced to retreat into his Neumeister persona more and more as life as his real self becomes increasingly unbearable.
A tense psychological thriller, This Sweet Sickness unveils an obsessive personality whose mad world becomes increasingly chaotic as it increasingly deviates from reality and ultimately impinges in a fatal way on the lives of others around it.