In the Guise of Mercy by Wendy Hornsby.
As one of his last requests, Maggie's late husband detective Mike Flint asked her to find out what happened in one of his cold cases. A young gang member, Jesus Ramon, who also happened to be one of Mike's informants, simply vanished one day after Mike dropped him off. Everyone assumed that Jesus, who was an addict and a small-scale pusher, had died of an overdose or been killed in a gang disagreement and the body quietly disposed of.
But Maggie is determined to fulfill Mike's wish, to give her search some credibility, she decides to make it the focus of her next television special. Delving into Mike's files and talking to his old colleagues, Maggie realizes she's onto something bigger than she expected when someone breaks into her house.
The descriptions of documentary filmmaking and the grittier side of Los Angeles are wonderful, and Hornsby is an amazing writer. It's good to see her (and Maggie) again.