Hilda: Hilda and the Mountain King (Hilda #6)

We rejoin our heroine for her latest adventure just as she awakes to find herself... in the body of a troll! Her mum is worried sick, and is perplexed by the strange creature that seems to have taken Hilda's place. Now, both of them are in a race to be reunited before Ahlberg and his safety patrol get the chance to use their new secret weapon to lay waste to the trolls, and Hilda along with them!