William Mumler the spirit photographer

Though I haven't seen this book, it looks like a great contribution to the literature of ghost photography: The Strange Case of William Mumler, Spirit Photographer by Louis Kaplan, University of Minnesota Press, 2008. Here's a link to a Google search that will lead you to various Web sites about the book. The virtual American Museum of Photography also has a great exhibit on spirit photography called Do You Believe?: A Ghostly Gallery.