I stumbled on this when my kids were young and have loved it since. Its called Santa's favorite story. Santa tells the story of the first Christmas and explains that that is why he does what he does.
My favorite has always been, and probably will always be 'Twas the Night Before Christmas. I have it memorized so I don't even "need" the book anymore. It's a family tradition to read on... the night before Christmas... Imagine that! hahaha
My favorite is one I had when I was little called The Sweet Smell of Christmas. It is about a bear family getting ready for Christmas, but it is a scratch and sniff book so you can smell candy canes, hot chocolate, an orange, pine trees etc. A few months ago, I was so excited to see new versions of the book are still sold on Amazon, so I bought it along with a scratch and sniff version of the Night Before Christmas.
I'd have to say The Night Before Christmas as well. Nothing can replace the memories I have of reading it to my daughter every year, in all it's varieties.