Here is an awesome website for all the comic fans out there: COVER BROWSER.
Cover Browser is a massive database of over 450,000 comic covers, starting at, well.... the beginning I guess. You'd be hard pressed to think of a popular comic that you can't find on there.
Some of the scans are a little low-rez, but that's OK. For me, they mostly serve as inspiration. When I start to get frustrated with a book cover assignment, telling myself that it's the vertical format that is preventing me from excelling, I simple take a look though this site... where there are half a million examples proving otherwise.