Killing the Story by Joan Livingston
Isabel Long is a former journalist who now works as a private investigator. Both jobs play to her strengths; she's curious, tenacious, tough, and determined. This is the first of the series that I've read, but it worked fine as a stand alone. Isabel is hired to investigate the death of a local newspaper editor whose death was originally ruled an accident.
I liked Isabel. She's older than most of the main characters in mysteries I read, especially the women. She's probably in her late 60s, has silver hair, but she's smart and funny and attractive. Her mom is her sidekick and the interactions between the two are well-done. They have a close relationship but they both realize the other is strong and smart and capable. Isabel doesn't have to hide anything from her. Isabel has a boyfriend who manages to worry about her without being over-protective.
There are several reasons to believe that Estelle's death was not an accident...