Edit: Although it was changed for the blurb, it seems that 'irresistable' may be a legitimate spelling after all.
Transcript follows. Many thanks to the submitter, who wishes to remain anonymous.

Transcript
HARLAN ELLISON
12 February 92
Dear Amy:
You may use this on Connie's new book. In full, no ellipses, no altering for any edition. And send me a copy of the book, please. And warn the troll drones in your art and publicity departments that if they fuck around with the quote, change it or alter it for a meaning not intended, I will come and find them and kill them so dead I'll murder their ancestors!
Charmingly,
(Signed)
"The writing of Connie Willis is irresistable. Clever, cunning, and capable of tantalizing the most demanding reader."
HARLAN ELLISON