Breakdown by Jonathon Kellerman

The story starts with pyschologist Alex Delaware being called in by a psychiatrist friend to consult on a case. The psychiatrist is treating an actress with mental health issues and he wants Alex to determine whether she is competent to take care of her 5-year-old son, Ovid.  Flash forward a few years and Alex is once again called in. The psychiatrist has died, the now former actress is in sorry shape, and her son is nowhere to be found. So Alex and his cop friend Milo start trying to untangle what happened to cause Zelda Chase to spin so out of control--and where her son is.

It turns out to be a very convoluted case with more victims than expected and some unexpected villians. All in all, a gripping read.