He mentioned this back in March at that lecture/book signing I attended. I think its going to be all of the recipes from the show. Probably up to last season, I'm guessing since he's working on it now and the current season just started. Don't know what else he'll have in it.
Normally, I would say you can just download the recipes from FN's site. But... its Alton! I'm going to buy it.
AND and another chance to attend a book signing!