BTFB, I think there's a decent history of the truce itself published by Pan - called the Christmas Truce, or similar. I got my copy on their budget military history imprint, and it pops up fairly frequently in publishers' outlets like "The Works", worth watching out for.
Also of interest might be Tony Ashworth's "Trench Warfare 1914-18 - The Live and Let Live System". Ashworth's theory is that warring sides developed a system allowing both to maintain the appearance of aggression, whilst keeping actual violence at a fairly low level along certain sectors of the front. I think he talks about the Christmas Truce in the book too. A little bit on
Wikipedia mentions this book.