My friends Matt Alt and Hiroko Yoda have written a really entertaining book about Japanese mascot characters called Hello, Please! The book explains the context for why mascot characters are so prevalent in Japanese society, as well as dozens and dozens of photos of these characters, promoting everything from earthquake safety to regional teas to proper digestion.

The Chronicle Books page for it is here, and there's a video of Matt on a mascot safari here.

I strongly recommend this if you're into Japanese culture, and after reading it you will definitely have a trained eye for spotting these mascots in the wild.