My friend and fellow Spenser fan Mark Luebker stumbled upon a TV Guide article explaining why Susan Silverman (as played by Barbara Stock) left Spenser: For Hire after the first season, then returned for the third and final season.
The year Spenser: For Hire premiered, a story arc reached its climax in the Spenser books. After separating from Spenser for two books, Susan finds her new lover is trouble. She calls Hawk for help and, only as a last resort, calls Spenser. This was Parker's fictionalized replay of his separation and reunion with Joan.
I thought the show was simply mirroring the books. In interviews, Parker had said Stock was busy being a mother, but according to TV Guide the show needed better ratings and the writers didn't see much potential for Susan as a character.
Happy couples like Spenser and Susan mean little if any conflict or sexual tension. Robert Parker might succeed in making the relationship a background feature because the books are mainly about Spenser. Television viewers begin to wonder why an actor is standing around. Doesn't she have more to do?