I am loathe to jump into this, but, well, no I'm not really....
What kind of Attorney-Client privilege would ever exist between a book writer and his publisher? If you mean between McGinness and the Random House lawyers, how could McGinness waive "attorney-client" privilege that exist between Random House and it's lawyers -- they certainly weren't McGinniss's lawyers.
I'm sure there is SOMETHING that he screwed up by talking about the existance of letters, but I don't see how it is "attorney-client privilege", unless you were talking about McGinness and HIS lawyer, not the Random House lawyers.
I would think that letters sent from McGinness to his publisher would never have been covered by any particular privilege, but that his revealing them gives Palin a reasonable expectation that they exist, which would allow her lawyers to get a judge to order them turned over.
Duh! When McGinnniss signed on with RH...their lawyers were involved with him and the book, legally. he thus opened all legal avenue when he talked to a third party about the process