What to get a girl for their 20th birthday?

What would you get a gift for her 20th birthday? We are having some trouble with ideas. She's so sweet and always gets everyone a gift for their birthday but when it comes to her birthday, she always says she doesn't want or need anything. Her family get her the more expensive gifts so she wouldn't like for everyone to get her expensive things.

She doesn't wear makeup or cook so anything in those categories are out of the question. She's not a party person either. She loves reading books and she is also starting to love getting her nails painted. Would getting her a book she's been wanting to read or nail polish be an acceptable gift?