Some helpful books about hell

A while back someone asked for my reading list, so here are some of the titles that I read specifically for Hellbound? Of course, who can say what other books have shaped my thinking (such as the Bible, which is not included in the widget below). But these are books I read specifically to prepare myself to make this film. I'm going to compile as many articles, web pages, blog posts, etc. as I can. Unfortunately, they don't fit neatly into an Amazon widget.