Warren Ellis is a big name in comics, and he's published one other novel, Crooked Little Vein, which I mentioned here. This book is nothing like that one.
In this one, John Tallow, a New York cop, and his partner are sent to a domestic disturbance that goes bad. Really, really bad. In the course of the investigation, the cops find a room that's filled with guns. Hundreds of guns. And, as it turns out, each appears to be tied to an unsolved crime.
So the CSI investigators get involved. These are not CSI types like you might see on TV. These are some strange people. So is John Tallow, for that matter. He's already somewhat odd, and his partner's death doesn't help things any. But he's the one stuck with the investigation. What it leads to is a conspiracy of long standing and a killer like nobody else in fiction. I'm not giving anything away here. We meet him fairly early on. We meet some other odd folks along the way, too.
If you're looking for another Crooked Little Vein, this isn't it. If you're looking for a police procedural like no other, you've found it.