The decorations would include a silver throne for Santa surrounded by poinsettias - maybe this is where I would sign books.
In terms of food, there are a few things from the book that are memorable. There are pizzelles, which would be cool to serve because not everyone knows about them. (Pizzelles are super-thin cookies - think hard crepes - that are made in a kind of waffle iron. I'll take a picture of some next Christmas. My family makes them every year, which is kind of funny because pizzelles are Italian and my family is not. Nonetheless, they're a personal Christmas tradition and one that the family in my novel shares.) Candy canes should definitely be served, preferably by an elf, and lastly, Famous Elven Oranges. These are supposed to be the biggest, juiciest oranges in the world. In the book, they're only grown in greenhouses at the North Pole, and each good child gets one in their stocking every year. Ideally, I would get custom golden labels made that I would put around each orange.
Speaking of stockings, they would be cool to give out as favor bags, but I have a better idea. In the book, Santa delivers all of the children's presents in gauzy white gift sacks with red drawstrings and the child's name stamped on the bag in red ink. These would be beautiful - especially if they were under a Christmas tree at the party - and relevant to the story.
So there it is! My fantasy book launch party. It would be perfect if it took place sometime between Thanksgiving and Christmas, but I would take such a lovely party at any time of year.