Our Gods Wear Spandex: The Secret History of Comic Books, by Chris Knowles. Summary: Was Superman's arch nemesis Lex Luthor based on Aleister Crowley? Can Captain Marvel be linked to the Sun gods on antiquity? Author Christopher Knowles answers these questions and brings to light many other intriguing links between superheroes and the enchanted world of estoerica. Occult students and comic-book fans alike will discover countless fascinating connections, from little known facts such as that DC Comics editor Julius Schwartz started his career as H.P. Lovecraft's agent, to the tantalizingly extensive influence of Madame Blavatsky's Theosophy on the birth of comics, to the mystic roots of Superman. The book also traces the rise of the comic superheroes and how they relate to several cultural trends in the late 19th century, specifically the occult explosion in Western Europe and America. Knowles reveals the four basic superhero archetypes--the Messiah, the Golem, the Amazon, and the Brotherhood--and shows how the occult Bohemian underground of the early 20th century provided the inspiration for the modern comic book hero.
I haven't read this, but it's been recommended by several people I know.
Holy Superheroes! by Greg Garrett. Summary: Spider-Man. Batman. The X-Men. The Fantastic Four. Comic books and the characters they have spawned have become twenty-first-century mythology. Greg Garrett helps us see the profound depth that can be found in the glossy, fast-paced, and often violent world of comics, graphic novels, and the films they inspire. The book features extensive discussions of Spider-Man, the Fantastic Four, and the X-Men. It includes an appendix with descriptions of twenty-five comics and graphic novels Garrett recommends for discussion of spirituality and comics.
This one I've read, and enjoyed. Granted, much of the spirituality in this comes from the Judeo-Christian schools of thought, but a lot of it can be applied to most religious belief systems.
The Science of Superheroes, by Lois H. Gresh. Summary: The Science of Superheroes takes a lighthearted but clear-headed look at the real science that underlies some of the greatest superhero comic books of all time, including Spider-Man, Batman, Fantastic Four, and many more. Each chapter presents the story of the origin of one or more superheroes and asks intriguing questions that lead to fascinating discussions about the limits of science, the laws of nature, and the future of technology. If gamma rays can't turn a 128-pound weakling into the Incredible Hulk, what could? Are Spider-Man's powers really those of a spider? Could a person ever breathe water like a fish? From telepathy to teleportation, from cloning to cosmic rays, this vastly entertaining romp through the nexus of science and fantasy separates the possible from the plausible and the barely plausible from the utterly ridiculous.
I've read this, and it's pretty good if the subject interests you.