Definitely be careful with the Twilight books. A lot of people I know want nothing to do with them. That's a gift I would ask about! (I know I am totally uninterested in that series!)
I usually ask for cheap jewelry and a few clothes, perfume (if you know what she wears, I tell my mom), letter shirts for my sorority, makeup kits (from places like Sephora, Ulta).
I've gotten sporting event tickets several times for holidays, concert and musical tickets too. But again, that is something you may want to try and fish out of her (for stuff she is interested in).
I'll try and think of more. It's harder to think of things to ask for as I've gotten older, at least I've found. A lot of the things I get anymore are things I have specifically asked for.
The best way to get information is to take advantage of opportunities of the conversations coming up. What I mean is, if she sees something and she seems interested, ask as much as you can about it, without being too obvious! I actually went to Super Soap Weekend for my mom who couldn't go, as a total surprise, for her Christmas present (make a big scrapbook). I asked her a question about it, and even got her to say who she wanted to meet! It made my job SO much easier!
Good luck!