Maybe its because I'm still recovering from this damnable cold (shakes fist futily in the air), but there were large parts of this story I just didn't understand.
Tim Powers short story deals with a man (the titular Pat Moore) who is haunted by his wife's suicide (also the titular Pat Moore). In addition, this wife ends up haunting him as a sort of guardian angel in order to protect him from a third Pat Moore. This third Pat Moore has discovered the key to immortality, which involves becoming a ghost and then taking over another Pat Moore's body. The plot also involves some weird physics that I don't even pretend to understand, the idea that ghosts exist outside of linear time (and thus exist at every point and every place at once), and a chain letter. See what I mean?
Ultimately, the central thrust of this story, once you strip away all the creepy, confusing, surrealness of it, is to explore Pat Moore's coming to terms with his wife's death and his possible role in her suicide. This part of the story is quite touching and human, and acts as a great counterpoint to the creepy, insane Pat Moore ghost.
Overall, this is possibly the strangest short story I've read so far in this challenge, although a few others come very close. Serves me right for reading so much Science Fiction and Fantasy of late.