As above, really. She always has a couple of books in her stocking to read over Christmas. She's an obsessive reader with a well stocked school library (and these days I really don't know what she's been reading), so it needs to be a bit off beat and unlikely to be in the library.
Non-fiction might be good as she's less likely to pick those up herself - last year I got her the Little History of Science which went down well. Auto/biographies might also go down well, esp science/nature-y ones.