Del Árbol routinely accumulates European crime-writing prizes, but “A Million Drops” is an effort that defies categorization, pulling together the best elements of historical fiction, psychological thrillers and literary character studies. The book begins with the murder of a young child in the early 2000s. But the killing at a lake outside Barcelona is only a launching-off point for del Árbol to unspool a dizzying multigenerational drama that begins in the years before the 1930s Spanish Civil War.