Lockwood and Company by Jonathon Stroud, there are four or five books. About an alternative world where ghosts are everywhere and an agency that deals with them. Especially good as maon character is teenage girl who really kicks ass. They are fantastic and really quite creepy in places.
These are great! Thank you! She's just finished 8 Agatha Christie books and is a real book worm but she's obsessed with creepy stuff now and I didn't know what to go for. Will check them out and get her a couple for Christmas
Moondial - Helen Cresswell. The Haunting - Margaret Mahy
Depending on exactly what her tastes are: The snow spider series - Jenny Nimmo They are fantasy rather than ghost but they definitely have a creepy air about them, especially by the end of the series. The Dark Is Rising series, starting with Over Sea, Under Stone. Susan Cooper. Again, fantasy more than ghost but a fantastic set of books.
Mystery Horse by Jane Smiley. It's for "young adults" but I've read it. Nothing sexual, a lot of horsey terminology and quite Christian-based, but it's based around a horse whose owner has died and the new owner keeps seeing the ghost of the old owner... or something like that.