If you're willing to think it might be a true ghost story -- as the author, Gary Jansen, insists it is -- it's entertaining and eerie, with a few creepy touches.
If you think all ghost stories are bunk, though, you can give it a miss. This is not the tale of a literary haunting or an exploration of psychology. What philosophy and theology there is is beginners' philosophy and theology about ghosts.
Not that there's a great deal of advanced philosophy and theology about ghosts being written. But, the title notwithstanding, Holy Ghosts is not, at bottom, about the intersection between religion and ghosts. It's about the intersection between a religious man (who happens to be Catholic) and ghosts.
Granted, Jansen writes about his year-long attempt to learn what Catholicism teaches about ghosts. The problem is that Catholicism doesn't teach very much about ghosts. There's not much beyond, "It could be a ghost," before you're just retelling pious ghost stories.
And yes, there are a few pious ghost stories retold in Holy Ghosts. But their moral is simply, "Boy, there sure are a lot of pious ghost stories."
Now, a non-Catholic reading this book may well choke on all the Catholic stuff I'm saying is incidental. But while it might read like a different book if it were written by a devout Methodist rather than a devout Catholic, I don't think it would be a much different story.
Most of the answers Jansen gets to his questions about ghosts come, not from the Magisterium or even theologians, but from Mary Ann Winkowski, a self-described paranormal investigator who inspired the TV show The Ghost Whisperer. She is Catholic; she also preaches the "Law of Abundance" and astrology. (The book mentions the first part, but not the rest. Special bonus for skeptics: Jansen was editor of one of Winkowski's books for Quality Paperback Books.)
Not that Winkowski says anything contrary to the Faith in Holy Ghosts, but "a Catholic says X" is not the same as "Catholics say X," much less "Catholicism says X."
Having groused for six paragraphs, I repeat what I said at the beginning: This is a ghost story, and a pretty good one. While the Catholic's world may contain ghosts, though, it is not consumed by them. If Catholics today need a lesson in the reality of the spiritual world, better to do it with examples of things going right (i.e., saints and angels) than things going wrong.