George R. R. Martin has now sold more than fifteen million books worldwide, and his readership will likely multiply exponentially after the launch, this month, of “Game of Thrones,” a lavish HBO series based on “A Song of Ice and Fire.” He is committed to nurturing his audience, no matter how vast it gets. “It behooves a writer to be good to his fans,” he says. Still, a close relationship with one’s audience has its drawbacks. As Martin puts it, “The more readers you have, the harder it is to keep up, and then you can’t get any writing done.”