Probably not many people have ever heard of or read The Dark Tower by C.S. Lewis. It might be because the book isn't finished. The original manuscript was never finished because Lewis started it right before his death. The manuscript was also damaged by handling and thus is missing a few pages.
The Dark Tower probably began the now popular fiction topic of parallel universes. It tells the story of a scientist who has invented a mirror that will show the viewer a view of a parallel universe. Eventually, one of his colleagues falls into the mirror and into the parallel universe where a horned man is ordering slaves to build his tower. The man was angry because he saw an alternate version of his fiancee.
He eventually finds a library in the alternate dimension and begins reading about theories of parallel universes. The manuscript cuts off at this scene.
My speculation is that either C.S. Lewis was tired or he didn't know what to do next because his theories about parallel universes were running in circles. It was getting a bit too obscure. I don't really know how he would have ended it. I've heard about people coming up with ends, however.
The best thing about this book is that it hasn't been tampered with yet. No one ever added anything to it, but that's probably because Douglas Gresham, Lewis' stepson, won't allow anyone to do that.
This book is interesting if you can find it.
I'm afraid I can't rate it because it is unfinished.