I was thinking about a book for the more scientific person on your Christmas list this year. Santa Lives! by Ellis Weiner would be the perfect choice. It offer five different arguments about the existance of Santa. (Like we need to prove that anyway! Of course he does.)
Although being the editor from National Lampoon, Weiner gives the reader a solid basis of proving that Santa is real. For example, take the causal argument: What causes christmas presents to appear around the world under the tree at the same time? Santa, of course.
Each argument is elaborately discussed, with additional information given to back up each argument. So, pick up this book and successfully argue when some non-believer says that Santa doesn't exist.