What is a book that no one has heard of but changed your life?
Non-fiction: The Confessions of St Augustine
Not necessarily a book that no one has ever heard of, but I think too few people have actually read it. This book gives a glimpse into the heart and mind and life of one of the greatest church fathers ever. It is semi-autobiographical, but it's so much more than that. It's hard to describe. Make sure you get a good translation though. I recommend the version translated by Philip Burton.
Fiction: Out of the Silent Planet / Perelandra / That Hideous Strength
Here is a list of books that almost no one seems to have heard of, written by a very famous author, C.S. Lewis. This is his space trilogy and it is amazing. The Chronicles of Narnia are obviously written for kids, but they can be appreciated by adults. The space trilogy is kind of the opposite. It seems to be written more for adults, but kids could still read and enjoy it. I don't understand why this series has flown so low under the radar for so long. I recommend it every chance I get.