It’s from a comic called Sinfest. I think mostly web based, though I think it may be published in some places. The character’s name is Criminy, and he builds his book forts to hide from the world. Though lately in the strip one of Satan’s sexy henchgirls is falling in love with him, and bringing him a little bit into the real world. It’s a fun little strip: http://www.sinfest.net/