learning

I love to learn. Some of this I do formally through University, some my own reading, and recently via a set of CD's I picked up from www.teach12.com.
Recently I picked up a series of teaching called "Great Ideas of Philosophy" and am finding it an incredible experience to delve into the great thinkers and their views about reality.
I never knew there was so much written about seemingly useless thoughts like, "How do I really know I exist?"
As I get older I'm more and more finding that I like to read things that challenge my particular paradigm of the world.
It's easy to get into the habit of only seeking out information that fits my own perspective. That is to read Christian books, magazines, websites etc. That's not wrong per see, but when it is all I read then I fall into a partern of easy thinking rather than exercising my mind to get a wider perspective.
What book or course could you take to widen your perspective?