I'd get her Great British Bake Off: How to Bake: The Perfect Victoria Sponge and Other Baking Secrets as it's got everything a home-baker needs and then some. It's barely been out of my kitchen since I picked it up.
Second choice would be either Mary Berry's Baking Bible as it's a classic, or my new favourite The Boy Who Bakes, which is an impressive debut from the chap who won the first series of Great British Bake Off.
Then you've got the The Primrose Bakery Book and The Hummingbird Bakery Cake Days: Recipes to make every day special which are both decent choices, albeit slightly swayed towards 'fancy' cakes and could be too specific.
And lastly there's Baking Made Easy, which offers a nice varied range of bakes and has a large selection of new things to try out. Highly recommended, that one.