Image courtesy of magazineart.com
The Internet Archive is a great source of antique, public-domain cookbooks online. I love browsing through one or two when I have a few extra minutes, and their online view is a great reading experience: all the fun of turning the pages without the worry of messing up an antique book.
As TJ commented in the last post, these older cookbooks are much harder to cook from. I like to cook from books from the '30s and on. Thrift stores have plenty of vintage cookbooks, especially from the '60s and '70s. I find the best source for books from the '30s through the '50s is Amazon. Lots of people sell used books there for modest prices.
My very favorite old books to cook from, though, were my grandma's. Here are some recipes from her books that I've shared in the past:
Gingerbread upside-down cake
Do you cook from old books? Where do you get your books and recipes? What changes do you find you have to make to older recipes?