Nineteenth Penguin printing.
154 pages, plus 6 pages advertising other Penguin books.
Cover by Mark Salwowski.
First published by Michael Joseph in 1968.
Published by Penguin Books in 1970.
Reprinted 1971, 72, 73 (twice), 74, 75, 76, 77, 79, 80, 81 (twice), 83, 84 (twice), 86, 87, 88.
Story first published in a much shorter form as a novella in Amazing Stories, Vol 37, No 3, March 1963