This is the first American paperback edition. First serialized in the British magazine SF Impulse (1966/67), then published in book form in the U.K. by Sphere in 1969, and later that year in the U.S. by Berkley (Paperbacks). A revised edition was published in 1977 (hardcover by Harper & Row, paperback by Sphere [U.K.] and Dell [U.S.]). The text was further revised in 1985.