At Too Busy Thinking About My Comics, Colin Smith wrote a four part exploration of All Star Superman and Superman: Earth One. It's really, really good. Part 1; Part 2; Part 3; Part 4. In-depth, clear, and insightful.
Here's an article about superman's ethics.
This one's from the Superman Homepage: (Not) Believing Again: Superman Returns--From Postmodernism to Faith (and Failure) by Ben Murane. It's an academic approach.
Speaking of an academic approach, here's Brent Allred's thesis from Melbourne College of Divinity, it's about the use of movies and literature in Christian Education of kids, it's got some stuff about Superman.
And another thesis, this time about Superman and America, by Michael Mautner.
That's enough of that for now.