We've been opening one book a day, usually at night just before Charlie goes up to bed. I think he likes the process because he will now point to the mantel and say "book" each evening.
We may go with a more traditional advent calendar in the future if I find one that is really appealing. But the book calendar has been a fun way to mark the days and also continue to encourage Charlie's reading. I'm already looking forward to having a bigger collection of holiday books next year to wrap and unwrap throughout December.