Book about Gacy
If you are interested enough to read about him, I recommend "Buried Dreams: Inside the Mind of a Serial Killer" by Tim Cahill. It's pretty much written from Gacy's perspective.. let's you see how he viewed himself (as a victim, of course) and the world around him. There is a section later in the book that walks the reader through one of his nights with one of his victims, and it is very disturbing to say the least.
A good read, if you like to be disturbed.