Maybe a gift certificate to a bookshop, either bricks-and-mortar or online, with a note that this is for a special cookbook she might like (or anything else, for that matter). Another option might be a magazine subscription like Gourmet or Bon Appetit, or a Martha Stewart one on food/baking.
I get a kick out of looking at old, old cookbooks. There are lots listed on Amazon.com - for example, if you could locate an old copy of Fannie Farmer's Cookbook, it makes for great, nostalgic reading.
Seaturtle