As Lewis explains in his preface, The Great Divorce is a response to the attitude of William Blake, among others, that someday there will be an ideal union of the secular and divine, or Heaven and Hell. Unable to see how this could ever be, Lewis wrote The Great Divorce to explore the issue further. The result is a religous allegory of the highest caliber and impeccable, nearly frightening, insight.
As with all allegories, The Great Divorce relies on symbolism to make its point. Lewis admits his book is not to be taken literally as a tour guide of either Heaven or Hell, but merely as an artistic expression of his ideas. And the plot device works well--Lewis's intensely brilliant yet readily accessible and familiar writing style makes it easy to imagine oneself in the narrator's shoes on the uneasy bus ride out of Hell, hunkering under the great mountains of Glory, or even talking uncertainly with Ghosts or Spirits (oh yes, there is a big difference!).
The Great Divorce is a fine read for anyone, believer or unbeliever. The former will find many challenges and assurances; the latter will discover new insights into the Christian faith not readily available from other sources. Above all, The Great Divorce is a Dante-esque tour of not only the Worlds Beyond, but just as importantly one of the often unexplored metaphysical World within us all.