One of the first books that gave me hope, was the Tao Te Ching written approx. 6th century BC. A good friend gave me a copy for my 21st Birthday and I still have a copy with me.
The first thing that struck me, when reading it, is that this book which this master did not want to write. I've re-read it many times since then and I never cease to be astonished at the contents. Basically he manages to write about the unwritable. The content refers constantly to what is inside a person - the universe! His teachings refer to "The Way". This can be visualised by looking at the symbol for Yin and Yang and consider walking along the dividing line. It's almost impossible but that is what is required. In the Christian Bible, there is a quote which points to the same way:
Matthew 7:13-14 New International Version (NIV)
“Enter through the narrow gate. For wide is the gate and broad is the road that leads to destruction, and many enter through it. 14 But small is the gate and narrow the road that leads to life, and only a few find it."
Lao Tzu refers constantly to the Nothing which is Everything. Learning the meaning of that took me many years and learning to practice that is something that I still/will have to practice for the rest of my life.
As an example :