December used to be a great month for digital sales, though January was much better. People got Kindles for Christmas and spent the next couples of weeks buying books for it. That effect has now pretty much vanished. It's hard to judge by 2017, because the reports have been so unreliable, but December to date has been my worst month in a couple of years. My sales always slow or stop over a holiday, and this is the mother of all holidays.
My paperback sales have always been good in December, but that too has come to a screaming halt because almost all of my books are shown as backorders on the Amazon store, anywhere from 1-2 to 6 days before shipping. At the moment, my 16 paperbacks constitute about 35 percent of my book sales, while the 26 Kindle editions account for 65 percent.
For four of those books, the paperback version has out-sold the ebook. This often happens. (Three of them are non-fiction, which may account for it.)