The stories in this collection are set in the same world and some characters recur, but are otherwise unconnected. The basic premise is that god-like ancient magic users secretly live in the modern world, all building their own power bases and fighting for supremacy (or something like that: it isn’t really explained). When these guys come in contact with normal humans, the results are usually bad for the humans.
The writing is very good, and for me this was the most enjoyable aspect. The title novella Coldheart is the strongest story for my taste. The rest are quite readable but I found them a tad depressing. I like the occasional happy ending, or at least the sense that something might get better for someone. If you are a fan of dark and grim, this book is well worth a try.
Expect hearts being ripped from chests and other violence. Thankfully, not dwelt on beyond my tolerance for squick.