"Classic" is the word generally used to describe a work that is still fresh and relevant years after its first publication, be it written millennia ago like Plato's dialogues, or centuries ago like the crisp and startling words of Thomas Jefferson or Teilhard de Chardin, or just three decades ago as was the first edition of The Crack in the Cosmic Egg.
Classics, in addition to having survived the years, are also viewed as such because they've changed not just a few individual lives but entire cultural paradigms. With this book, Joseph Chilton Pearce introduced a crack into the egg of Western culture in the early 1970s that in many ways led directly to the more esoteric aspects of the modern self-help movement, the widespread interest in mysticism, and the discovery of the intersection of science and spirituality.
This is a life-changing book, and, as such, it's worth taking in slowly and comfortably over some time. I remember it took me months to completely digest it the first time I read it, as I kept encountering concepts or ideas that I had to reality test, or discuss with others, or carry around, look at from different directions, talk with myself about, and even dream about before I could fully understand and integrate them into my life.