This is the third volume in The Collector series after Dead Harvest and The Wrong Goodbye. If you haven't read either, you may find this review a bit spoilery, in which case stop now. (In that case I'd also recommend reading the other books first: if you read this book you will probably want to read them anyway, and although this part is perfectly good as a standalone, it does give away a fair amount about their plots).
Sam Thornton is The Collector - bound, after agreeing a Faustian bargain, to the service of Hell, specifically, collecting the souls of the damned, under control of his "handler" the mysterious and glamorous being Lilith. In the earlier books, we learned how that happened, and some of the politics of Heaven and Hell, as well as the coven of rogue collectors called The Brethren. In this third volume, which picks up directly from the end of the second, Sam turns his attention to them.
It's noticeably different in concept to the earlier books: the plot is much less twisty, even more gore-filled and violent (with, I think, one of the most disgusting methods of travel to trouble the gentle reader of a horror novel: just wait) and with fewer plots-within-plots, reverses and revelations. But we have the same wise-cracking, hard-boiled Sam and the same paradox of what (despite his crimes) seems to be a basically good man forced to do Hell's bidding and feeling his soul die a little more with every bodily possession, every reaping he carries out.
The book is immense fun, as were the earlier ones, and I don't object to the gore. I've only marked it four stars instead of five because it seemed just a bit... well, plot light I suppose. Perhaps it's best to think of the three books as essentially one story, with this part really the action filled finale? It does feel that way, and I wonder if Holm plans more or if this is really the end?