Cold Spring Harbor, New York: Self-published 1991. First Edition. Typed letter from James D. Watson (Director, Cold Spring Harbor Laboratory) to Evelyn Fox Keller (then at University of California) on Cold Spring Harbor letterhead:
"Dear Evelyn, I would be pleased to talk with you and David Cohen about the Human Genome Program. But I am now hesitant to talk about my earlier days in science. Now I am working on a successor to The Double Helix. Until it is finished, i'd best not talk prematurely. Sincerely, James D. Watson". Signed in ink "Jim Watson".
Letter is folded for mailing, 5 1/2 x 8 1/2 inches with cellotape repair where staple was remove upper left. Verso blank. Signature strong. Near Fine. 
"James Dewey Watson KBE (born April 6, 1928) is an American molecular biologist, geneticist and zoologist. In 1953, he co-authored with Francis Crick the academic paper proposing the double helix structure of the DNA molecule. Watson, Crick and Maurice Wilkins were awarded the 1962 Nobel Prize in Physiology or Medicine "for their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material""
The successor book to The Double Helix Watson is referring to was probably "Genes, Girls, and Gamow: After the Double Helix" published in 2002.
Evelyn Fox Keller (born March 20, 1936) is an American physicist, author and feminist. She is currently Professor Emerita of History and Philosophy of Science at the Massachusetts Institute of Technology. Keller's early work concentrated at the intersection of physics and biology. Her subsequent research has focused on the history and philosophy of modern biology and on gender and science.
(information above from Wikipedia which has in depth discussions of both people).