The Unkept Woman by Allison Montclair
I'm slowly working my way (totally out of order) of the delightful Sparks & Bainbridge series by Allison Montclair. Gwen is trying to get herself declared legally sane to regain custody of her son, and her psychiatrist tells her no more murder investigations. So when her friend and business partner Iris becomes the primary suspect in the murder of a woman in Iris's apartment, Gwen has to tell her friend she can't be involved, causing their first rift. But Iris is her best friend and when Gwen realizes her life may be in danger, she gets involved anyway. Another excellent chapter in the saga.