Hilda and the Mountain King by Luke Peason

For the first time in paperback, 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 now has to deal with 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 trolls, and Hilda along with them!