Poor L. Frank Baum (of “Wizard of Oz” fame) desperately wants to be Melville or Dickens—to write substantial novels for adults. But this path is hard, slogging work fraught with writer’s block. Baum’s problem is that his Emerald City fantasies flow from his mind like a tsunami, and make him the most treasured children’s book author of his era. When “printing ‘Oz’ books is like printing money,” it’s hard to stop.