Notice the book that Petey, the comic-loving introvert, is holding in his hands? I'm almost positive it's supposed to be the graphic novel Smile, by Raina Telgemeier.
(Sure, the cover's a different color, but I chalk that up to an ignorant colorist—the strip was originally produced in black-and-white, after all, and I assume it was touched up just for this website.)
Somehow it makes me incredibly happy that the creator of this strip, Richard Thompson, is enough of a comics nerd to put in a reference to a truly excellent, semi-indie graphic novel into his strip, at a resolution that renders it extremely difficult to perceive. Bravo, Mr. Thompson.