Well, timely books usually get published really quickly -- well, they're "timely" for a reason. And it will be four months after the Olympics... they can probably work really fast. I know someone who writes the Star Trek/Star Wars books and usually they can go to market extremely fast with those books, too.
Also, yeah, he's probably either started writing it or already had a ghostwriter in place. I suspect much of the book has to deal with everything but the eight golds -- his background, his training, his races including the 2000 and 2004 Olympics. And they could just fill in the blanks about the Beijing games.