A book's theme is an idea that appears multiple times throughout that book, designed to ask the reader a question that is deep and sometimes deals with questions of right and wrong. Themes emerge as readers make their way through stories.
A book's plot, characters and literary devices are all tools for expressing the ideas that the author wants to communicate. After reading F. Scott Fitzgerald's 1925 novel "The Great Gatsby," one theme that emerges is the corruption that has eaten away at the American Dream. The central character, Jay Gatsby, appears to have created a life of independent wealth for himself, but he has done so by engaging in illegal activities, selling alcohol during Prohibition and handling some shady financial transactions.
A book's theme is an overarching idea that comes over the reader over the course of experiencing the book. Some themes are archetypal in nature, which means that they are central to the human experience and so appear in literature of virtually every time period. One of these is the hero's quest, a story line that appears in works as old as the Babylonian Epic of Gilgamesh and in works as new as Bernard Malamud's 1952 novel "The Natural."