Small Blue Thing - S.C. Ransom (2011)
Alex is with her best friend down at the Thames, celebrating sitting her last exams. While there she spots something hidden in the mud, and pulling it out finds an old silver bracelet with a sparkling blue stone.
After wearing it round for a while, she realises that she is hearing a voice of a mysterious young man, Callum who begins to tell her about her amulet, and how he is stuck in an imbetween world. As their friendship grows, she begins to wonder if she should get rid of the bracelet, especially when Alex finds her life at risk, and she feels confused not knowing who to trust.
This is part one of a trilogy, and I think I will look out for part two. It is a young adult book, and intended for the 11plus market, which makes me feel old, as I am somewhat older.