Lao Tzu's Tao Te Ching is the source of Zen Buddhism, and is probably the most broadly influential spiritual text in human history.

According to legend, Lao Tzu left China at the age of eighty, saddened that men would not follow the path to natural goodness. At the border with Tibet, a guard asked him to record his teachings and the Tao Te Ching is what he wrote down before leaving. Lao Tzu's spirituality describes the cosmos as a harmonious and generative organism, and it shows how humans are an integral part of that cosmos.
David Hintons superb translation is inviting and immensely readable.