Felix Castor is, effectively, John Constantine with the serial number filed off. Down-and-dirty Northerner living in London and making a precarious living through supernatural means, with bad consequences for his nearest and dearest. There are differences, of course: Constantine isn't a professional, and lives in "our" world, while Castor is an exorcist, and advertises his services in the phone book. And in Castor's world, everybody sees ghosts, since they all started appearing a few years back.
It's a supernatural whodunnit, effectively, with Castor pulled out of self-imposed retirement - the nearest-and-dearest thing (think Newcastle) - to do a simple job but which gets more and more complex.
It's actually very difficult to read this without imagining Constantine chain-smoking his way through the lead role, stepping out of the shadows with a swirl of trenchcoat. But it's still very readable, nevertheless. Carey's a smart guy, and can write an enjoyable story. I'd prefer some more intelligent characters (he can do them, as Lucifer shows, so why not here?), but I'll still be reading the next one.