Have any of the writers or artists stated why Shepherd Book is stepping on a snake on the cover of Serenity: The Shepherd's Tale? There is no snake in the graphic novel. Is it a allusion to Book overcoming adversity? It may be a reference to this Bible passage, but I doubt is as it doesn't really connect to the story:
14 So the Lord God said to the serpent, “Because you have done this, “Cursed are you above all livestock and all wild animals! You will crawl on your belly and you will eat dust all the days of your life. 15 And I will put enmity between you and the woman, and between your offspring and hers; he will crush your head, and you will strike his heel.”