The Screwtape Letters place christianity, and religion in general, firmly in the context of the twentieth century. The evil versus good battle is fought by a bumbling fool who is conceited enough to think that he knows best, and human (if that is the right word) enough to believe that he is doing evreything possible, and doing it correctly, whilst the world conspires against him.
The book is, however, as much a philosophical novel as anything else, as Lewis seeks both to entertain and to reach a conclusion to one of man's greatest questions. Of course, we don't have to accept his conclusion if we don't want to - that is still philosophy's greatest virtue over science. This does not matter, however. THE SCREWTAPE LETTERS is fun to read and whether you are looking for spiritual enlightenment, or just a good book, you could do a lot worse than this.
Being fairly short, you'll finish it quite quickly, but this effect is amplified by the ease with which you begin to read the book, and the difficulty with which you put it down.