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.
I'll look this book up It's long been a worry to military planners that people don't actually like killing each other. There was a famous example during the American Civil War when a musket was found with eighteen balls in the barrel - the soldier had pretended to fire each time. It's a particular issue now as all military training is about dehumanising the enemy, but when troops are in peacekeeping duties, a different approach is needed.
The book I read on the Christmas Truce was enlightening.... Both sides were really afraid that this was going to personalise the enemy, and there is evidence that for some time afterwards the troops were shooting and deliberately missing the enemy.
