I am currently re-reading "Meditations" by Marcus Aurelius. It's right up there as being one of my favourite books of all time, and have always gone back to it when I feel like I've lost my way a little.
I imagine what it must have been like for someone like Aurelius, on the battlefield when all of his troops were asleep, to be writing this document and trying to reflect on the human condition. His situation was the complete opposite of someone like Epictetus, a slave, and yet they were both dealing with the exact same issues.
Simple, meditative and poetic, I highly recommend this book to those looking to do some introspection and reflection.