"Whoever it was" was Anthony Boucher, a famous SF editor and critic at the time. Jack sought his advice, somehow, because he didn't seem to find anyone to buy the huge MS he'd written. Boucher told him it was much too long, and Jack reduced it, using a number of sheets of the original MS to make modifications, and all those original sheets eventually ended in a wastebasket. Give me a time-machine, tell me how to use it, and I'll go back in time to retrieve the whole lot...