You can zoom in on specific ghost story traditions, like OU ghost stories, Indian ghost stories, etc., or you can do a project where you combine ghost stories from different countries. There are ghost stories in cultures all over the world.
Research Tip: You can get an overview at Wikipedia, and you will find a List of Ghost Stories there too. Be sure to check out some related terms also like revenant and hungry ghost, etc. There are some general ghost books at the Freebookapalooza also: The Ghost World by T. F. Thiselton Dyer and The Book of Dreams and Ghosts.