I'm partial to free books in the Classics category and something short for the busy month of December, so I'll vote for Ethan Frome.
I have Cold Comfort Farm on hold at the library, but since I'm second on the list I'm not likely to get it in time for this month. I do have the audiobook for The Good Earth, so I'd be willing to try that if it wins. On the other hand, the pro-Christian theme of Quo Vadis is unappealing to me and I gave Crime and Punishment a try last summer but couldn't get past 50 pages, so I'd give both of those a pass. Dickens and Little Women are too long to tackle right now.