It's so hard to know what another person will consider "incredible". One of my favorite books is Mystic River by Dennis Lehane. On the surface, it's just a suspense novel. But I felt it dealt with so many other themes - the loyalty and loss of friendship from childhood to adulthood, the loss of a child, the struggle of a person between "right" and "wrong" and how people determine what that is... I just thought it was a lot deeper than "just" a suspense novel.
I just downloaded a copy of James Hilton's Lost Horizon from the library. I read it when I was a teenager, so it's been many, many years. But I remember it being interesting and thought-provoking. It was written in the 30s, so I'll be interested to see how my tastes have changed over the years.