David Foster Wallace, author of the novel "Infinite Jest," was asked by Rolling Stone magazine to cover John McCain's presidential campaign in 2000. That assignment became a chapter in his essay collection "Consider the Lobster" (2005); the essay has now been issued as a stand-alone book, "McCain's Promise." In a phone interview, Mr. Wallace said he came away from the experience marveling at "how unknowable and layered these candidates are." Mr. Wallace also answered questions via email about presidential hopefuls, the youth vote and smiley faces.