Doctor Death by Jonathan Kellerman

Dr Eldon Mate, aka Doctor Death, has been the bane of the Los Angeles DA's existence, the bête noir of all opposed to assisted suicide and the angel of mercy to countless "travellers" who have found their reward via Mate's dubious vocation. He's also turned up in the back of his van, attached to his own death-dealing "Humanitron" machine and too far away from most of his blood and a certain external organ.

Enter Milo Sturgis, LA's only openly gay homicide detective and for the 14th time in 15 years (1985's award-winning When the Bough Breaks through to 1999's Monster), enter also his good friend, child psychologist and LAPD consultant Dr Alex Delaware. Unbeknownst to Sturgis, however, is a potentially case-stymieing doctor-patient conflict of interest. The mother of one of Delaware's young patients' was the beneficiary (or victim, depending upon your point of view) of Dr Death's services. The father, Richard Doss, is firmly in the latter camp, giving Delaware ample pause for reflection:

There is no shortage of suspects and the investigation goes back and forth. The sub-plot dealing with Alex's young patient and her family is another layer - did Stacy's father kill Mate? Milo thinks so. Alex thinks it's possible, but he very much much wants to think not. Over all, Alex really wears his emotions on his sleeve in this one, his own obsessiveness making him unable to let things go.