A few decades ago some of the dead started rising as ghosts. Weres also start to appear, but in Carey's world, weres are animals that are taken over by a strong ghost who then forces the animal into a human shape. There's no explanation for why the dead have started to rise, although, of course, there's the usual suggestion from some quarters that it's the beginning of the end of the world (the theory is mentioned only in passing, not a focus of the book).
Some people can see ghosts but they remain invisible to others.
Felix Castor is on the sensitive end of the spectrum. 95% of ghosts are passive and Felix isn't sure what they are. He prefers to think of them not so much as the dead coming back to life for unfinished business but instead as more of an emotional smear repeating itself in time.
Castor is a natural exorcist. He uses music, usually a tin whistle, to entice a spirit near. He learns the shape of it, binds the spirit to his music, and when he stops playing the spirit vanishes. He's a little curious what that means, but mostly tries not to think about it and keeps it impersonal.
He gave up doing exorcisms about a year and half ago, after he tried to exorcise a spirit from his friend only to realize his friend was being ridden by a demon, not a spirit, and every nearly destroyed his friend's soul. Now his friend's soul is tangled with the demon and his friend is in a mental institution.
Unfortunately, he's running out of money and his landlady who is also his friend and a former girlfriend still in love with his demon ridden friend, is also out of money and about to lose the house. So he takes another exorcism job, something that should be simple enough, but of course is anything but.
Unfortunately, the story itself felt pretty flat. I couldn't get invested in any of the characters.
The world is interesting though. The way the book ended gives me hope that the next book might be more interesting and develop the world a little more.