The Meditations of Marcus Aurelius
Not sure why this is on here really, but I will say I picked this up about a year ago and started reading it and was absolutely blown away by how relevant it still is 1,000 years later.  
...Think about how grounded you have to be to turn away from the life of material fulfillment that is offered to you as a Roman Emperor and instead live a life of philosophy and the mind.  Wise dude, and it's worth reading what he wrote.