He probably didn't have that hard of a time tracking it down, since the book never existed to begin with. Neither does Jason, by the way. William Goldman has two daughters, both rather slim. And he was married at the time to a perfectly nice (non-shrink) named Illene. Incidentally, scuttlebutt has it that his two daughters asked him to write a story. When he asked them what the story should be about, one said "A Princess," and the other said , "A Bride," and hence the story was born.
So far, only the first chapter has been published, however, I believe he is writing the whole thing himself. If not, a lot of fans are going to be very disappointed. However, if you're jonesin' for a Morgenstern Fix right now, you can get "The Silent Gondoliers" at most major bookstores.
Someone told me that he made up the whole thing. He wrote so many pages describing himself trying to track down the original Morgenstern, but if he wrote it all, then did he make all that up too? And whatever happened to Jason?
Also, in the 25th Anniversary edition of The Princess Bride, it includes the first chapter of the sequel "Buttercup's Baby." Did he actually write a sequel, or just the first chapter?