How cool that your mom is interested in learning more about something you're passionate about! If my mom asked me the same question I would definitely recommend to her Outrageous Acts and Everyday Rebellions by Gloria Steinem. I love Feministing, (posted to Help! Feminist Books for my Mother?)
I'll second what erin_amelia said about Pink Think. It's an easy and interesting read and it would probably be good for someone of your mom's generation. And Margaret Atwood's The Handmaid's Tale is a good fiction read that can help (posted to Help! Feminist Books for my Mother?)
Manifesta by Jennifer Baumgardner and Amy Richards is a great feminist starter. Though it is subtitled "Young Women, Feminism, and the Future," I think it is a very accessable book for women (and men) of all ages. http://www.manifesta.net/ (posted to Help! Feminist Books for my Mother?)
I like the following: "To Be Real," by Rebecca Walker. "Listen Up: Voices From the Next Feminist Generation." In it, there is a wonderful piece about how feminism brought a mother and daughter back together. "The Mommy Myth." Jessica's "He's (posted to Help! Feminist Books for my Mother?)
Wow, so many great suggestions! Thank you all so much! I think I'll be able to come up with a very good, diverse reading list for my mom with all of your recommendations. (posted to Help! Feminist Books for my Mother?)
How serious a reader is your mom? If she can handle big piles of books, the thing to do would be to go through the history, wave by wave. Third-wave stuff can sometimes seem a bit nutty to the beginner (posted to Help! Feminist Books for my Mother?)