This weekend, Fri. Nov. 13-Sun. Nov. 15 is the 33rd annual Boston International Antiquarian Book Fair.
Hours are listed on the web site. I went last year, and had a fantastic time. In particular, I noticed a LOT of Mark Twain as well as Thoreau, Emerson, Hawthorne, Alcott, Melville-the usual New England suspects. It's amazing just to see these works up close and think about who bought them and read them and owned them, and how they're a part of history and literature. There were also quite a few bookshops from overseas, particularly Britain, but also France, and some of the Scandinavian countries.
I don't think I'll make it this year, since $8 just to see isn't worth it when I've probably already seen most of it and unfortunately can't afford to buy. But if you're a book lover in Boston, and haven't been, definitely go!