I love many books, but I just finished Out Of The Silent Planet by C.S. Lewis.
It. Was. Amazing.
Admittedly, it did take me a little bit to really get into it, but once I did, oh my! I was underlining passages right and left!
It is basically about a man who gets kidnapped, taken to another planet, and is forced to find his own way once there. But it is also thought-provoking, and eye-opening, and gave me tons to think about.
I would highly recommend it.
(image found via a google search)