Also, creating the index was a real treat. Material is wherever you attempt to look. I made an absolute pest of myself calling people and chasing information. I managed to get faxes of the police report and pictures after Dahmer's arrest, which was a real coup. Get as many books as possible and go to the references sections, a true wealth of information sources.
Local courthouses are good for some transcripts.
When I began the book I did not have a PC and I actually wrote the book with no Internet help at all. I can't imagine what it would have been like with that available. Manuscripts should be, of course, very legible and well organized. Don't expect to get them back. Find a cheap printer.