I saw the idea a year or so ago, and immediately started planning to implement it this year. Instead of a traditional open-a-door-each-day, or other type of Countdown to Christmas, this "calendar" consists of a stack of wrapped books, each one telling a tale about Winter or Christmas.
My stack of books is partially random, having accumulated about half the number needed over the last several years, gifts from friends, thrift store finds, hand-me-downs. The remaining books I chose based on reviews read online. I've read about other families who have the same tradition, but borrow the books from the library instead of purchasing them.
I figure that as the kids get older, we'll probably swap out some books for others, but a good portion of them are going to remain staples of this tradition.
If you want to see the list of book titles, I created a shelf over at Goodreads for you to peruse.