Susan Sullivan also does a good turn as Martha, Castle's mother. She reminds him to leave the past in the past. Dredging up old memories tends to bring nothing but trouble. I like her a lot. Sullivan adds the spice and some wicked humor to make this show not the typical fare.
Sooner or later, the focus should come back to Beckett's dead mother. The suggestion has been made it was a homicide, but the storyline has been on the back burner for a good while. Whoever did the deed is assumed a serial killer, although the evidence is a tad thin. Other victims were killed around the same time, but it doesn't mean it is the same person. Big cities will often experience multiple crimes in the same time period.