Little Gem and the Mysterious Letters by Anna Zobel

Little Gem is settling into her life at Ellsworth Pining with the help of Ghost Henry and her other friends. Everyone is preparing for the Midsummer Festival and Little Gem is in charge of the special effect for the Midsummer play. But when Little Gem's magic starts to go wrong and she receives several mysterious letters, Gem is worried that the festival is going to be a disaster! Will Little Gem be able to work out who is sending the letters and regain her confidence before opening night?