> if you were to go against your duty as the head of NASA, Carl Sagan isn't going to rise from the grave and smite you for your transgression.

It's like swearing something 'on your mother's grave'. That's not something done because zombie mothers will rise up to enforce anything. It's supposed to signify that you hold great reverence for your mother and that your conviction is as strong as that respect. Swearing on the grave of your mother and lying would be dishonoring her and betraying the reverence you supposedly held. Swearing on Carl Sagan's book is expressing that the new director has great respect for Sagan (or at least that particular work) and that she'll treat her oath with the same level of respect. I think it's pretty appropriate given the role.