If that's the kind of thing you are interested in, I suggest getting a copy of Harold Roth's translation of the Neiye chapter of the Guanzi. It's called "Original Tao: Inward Training and the Foundations of Taoist Mysticism." The text he translates is the oldest extant Chinese text on internal meditative training, and probably comes from the same lineage of practices that later produced the Daodejing. I highly recommend it to anyone with an interest in archaic Chinese mysticism.
I ordered it today. I don't know if they have even finished cataloging all the documents they found at the Dun Huang caves. There was so much stuff there it will keep a couple generations of scholars busy. Did this volume come out of that find or did it come to light elsewhere?