Bad Night is Falling by Gary Phillips

After a fatal firebombing, gangsters hire Ivan Monk to prove them innocent 

The Cruzado family wakes in the middle of the night, their lungs burning with smoke. Staggering out of bed, the father tries to rally his family to escape their house. When his daughter and mother don’t make it onto the lawn, he goes back in for them, never to return. This small housing development bungalow was supposed to be a new start for the Cruzados. Instead it became a tomb. 

The logical culprits are the Ra-Falcons, a street gang that holds sway over the Cruzados’ neighborhood. Only gangsters could be twisted enough, the police think, to toss a Molotov cocktail into a little girl’s bedroom. But when the gang’s leader hires private eye Ivan Monk to prove the Ra-Falcons’ innocence, Monk unearths a conspiracy far more sadistic than any violent gang.