Just finished re-reading G. K. Chesterton's The Man Who Was Thursday: A Nightmare.
I remember this being an entertaining book, but recent events in my life prompted me to go back and re-read it, especially for the last chapter.
This book defies genre classification. My best attempt would be to describe this book as a farcical spy novel, but it is well-crafted enough to be "literature" and still hold my interest. Wikipedia calls it a "metaphysical thriller" (although if you even plan to read this book, do NOT look it up on Wikipedia as the plot summary may spoil the entire experience for you).
I hope that I do not spoil anything by saying that - in the closing chapters - this book unexpectedly ends up touching on the deepest questions about (the Christian) God, the Purpose of Life, and the Problem of Human Suffering. In no way does it attempt to answer these questions, which is as it should be.