Chaim Potok is such a fab writer, try Davita's Harp or Asher Lev.
Roald Dahl's adult stories, Perfume by Patrick Suskind and Donna Tartt are all great to read.
Maybe The Wasp Factory, that's v. popular.
Nick Hornby with things like About a Boy, Juliet, Naked and High Fidelity, and Tony Parsons is sort of a pale imitation of him. Adrian Mole :D
There's always Colette (she might like the Claudines to start with) and Simone de Beauvoir...I'm reading The Mandarins at the moment...not heavy in anything other than a literal sense (700 pages). It is really, but you can read it on different levels, so I doubt it's too much for a bright 15 year old. Memoirs of a Dutiful Daughter is also rather nice to read.
Remains of the Day- Kazuo Ishiguro (sp?).
Another vote for Alexander McCall Smith- there's more than 10 books in his Ladies' Detective Agency series.
If she's prepared to read classics, then try the Tenant of Wildfell Hall- I was gripped at 13/14. Quite important book at the time too. If she's reading Jane Austen, Clare Tomalin's biography of her makes the whole picture clearer. I recently read it and liked it and I don't even read Jane Austen (only Northanger Abbey- seems easier to stomach than the others, but gah there is an irritating heroine!). She could try novellas like Death in Venice, absolutely beautiful. Another translation though.
Oh and The Invisible Man by HG Wells, a bit scary in parts.
Best thing is just for her to read about her interests though.