Dancing on the Head of a Pin by Thomas Sniegoski

With Madeline gone, Remy has very little left to enjoy in his life as a human. He's been ignoring his private investigation business, for the most part, and avoiding the few friends that he has. He and Marlowe are just getting by, tho Marlowe is probably getting over his grief more quickly.

When a Nomad, one of a group of angels who left Heaven voluntarily after Lucifer's war, turns up multilated in the hands of one of the Denizens, a group of "paroled" angels still doing penance on Earth after a stint in Hell, Remy gets the feeling that his life is about to get interesting once more. He visits the leaders of the Nomads to see if they might know what the Nomad was up to before he met his dismemberment and death, but they claim ignorance.

Shortly after that, Remy is contacted by a potential client who wants him to investigate the theft of some weapons from his extensive and expensive collection. Remy takes the job on a hunch and starts poking around. He consults his angelic friend, Francis, guardian of the gate between Hell and Earth, and finds out that the weapons may be The Pitiless, a set of weapons whose creation was inspired by angels, basically unstoppable, designed to kill over and over again.

More than one set of evil forces in the world are intent on getting their claws on  The Pitiless, and Remy is caught in the middle, trying to do the right thing, to prevent Heaven and Hell from going to war once more. Dark, brutal, somewhat depressing, but a good tale again from Sniegoski.