Seeking general 'ghost' information

Hello

I'm writing a song about a ghost that is clinging to a past lover and need some background information to give the piece the proper depth.
I once read on these forums about ghosts and how a spirit can get lost in limbo and as far as i remember it can come about through material attachments and emotional strains, but this is about all i can recall. I would like to know more about the nature of this realm further.
Any general information or links to comprehensive sources would be greatly appreciated.

Thanks!

Andrew