The nosh that began in 17th- century Poland and traveled around the world is now as common in American homes as apple pie. In this charming little book, The Bagel: The Surprising History of a Modest Bread (Yale Univ., $24),  Maria Balinska traces the history of the bagel from its humble beginnings to New York’s Lower East Side to freezers and bake shops from coast to coast. The debate, however, will continue to rage: is that square thing really a bagel? And why would anyone put blueberries in a bagel?