• Highly Recommended: Understanding Comics: The Invisible Art 
• The American Superhero: Encyclopedia of Caped Crusaders in History (EBook)
• The Cambridge Companion to the Graphic Novel (Print Book)
• The Encyclopedia of American Comics (Print Book)
• The Encyclopedia of Black Comics (EBook)
• The World Encyclopedia of Cartoons (Print Book)
• The World Encyclopedia of Comics (Print Book)

Web Based

• Comic Book Legal Defense Fund: The CBLDF is a non-profit organization dedicated to fighting censorship in comics. Their website is full of great information about the history of comics and the controversies that comics face today. 
• Comic Vine - Huge Wiki for comics
• DC Comics Database Wiki
• Library of Congress Art Exhibit: a beautiful online exhibition dedicated to the art of comics books. 
• Library of Congress Comics Collection: While most comics are not accessible through the LOC website, you can use this interface to see what the LOC has in its collections. 
• Marvel Comics Database Wiki