I was thrilled when my publishers, HarperCollins, asked me for another book – because I knew there was plenty more I wanted to find out about Peer and Hilde and their world. How would their friendship progress? What would Peer do with the ruined old watermill he'd inherited from his uncles? What were the trolls getting up to now? And of course the Nis was bound to reappear!

Also, I wanted to re-use some old northern legends about the seal people – not mermaids, but seals who can transform into humans by casting off their skins. I knew a story about a fisherman who married a seal woman, but was heartbroken when she deserted him and went back to the sea. What would that be like if it really happened?

Troll Fell is a winter book, set among the mountains. Troll Mill turned out to be a summer book, set along the wild Norwegian coastline – with mountains and trolls mixed in as well, of course!