Bruce Tinsley, creator of the conservative comic strip Mallard Fillmore, remembers feeling stunned when the fan letter showed up in February 1998. After all, his strip— featuring a right-leaning TV newsman or, more accurately, newsduck—was still in its relative infancy. Yet here was George Herbert Walker Bush declaring that he and Barbara turned to Mallard, “sage duck that he is,” first thing every morning. Even more gratifying, the former president thanked Tinsley for taking on “that horrible Doonesbury” and its creator, liberal icon Garry Trudeau, “a guy that tore me up in a vicious, personal way strip after strip.” By...