"Zen and the Art of Motorcycle Maintenance" and "Lila" by Robert Pirsig.
Also pretty much anything by Soren Kierkegaard. I'm fond of Simone Weil as well, but her writings aren't usually on the shelves at the local bookstore. There's also Thomas Merton, Thich Nhat Hanh, Paul Tillich, Rudolph Bultmann, Albert Schweitzer, and Wayne Teasdale. Of the older schools, Meister Eckhart and Jacob Boehme stand out.
ZV