Most of the time I read to be entertained, not to think. But some novels manage to do both at the same time, which is great.
Kaleidoscpe Eyes by Karen Ball introduced me to synesthesia, which colours (literally) the way some people see. I wonder if this is where the New Age ideas around auras came from.
This Present Darkness by Frank Peretti introduced me (and millions of other Christians) to the concept of spiritual warfare.
Confessions of a Teenage Hermaphrodite by Lianne Simon challenged the way I understood gender and challenges of birth.
Everywhere to Hide by Siri Mitchell introduced me to face blindness (which made for a great plot point for a suspense novel).