Of Worlds Beyond: The Science of Science Fiction Writing edited by Lloyd Arthur Eschbach.
Includes work by Edward E. Smith, John W. Campbell Jr., L. Sprague de Camp, Robert A. Heinlein, Jack Williamson, A. E. von Vogt, and John Taine.
Chicago: Advent Publishers, 1970. Second Edition, Second Printing. ISBN: 0911682058. 104 pp. 8vo. Includes index. Originally published by Fantasy Press, 1947. Very good. Paperback.