Knights of the Borrowed Dark (Knights of the Borrowed Dark Book 1)

Grey placed his finger in the middle of the shadow. 'What's this?' he asked. Denizen frowned. 'It's a shadow.' 'No, it isn't,' Grey said. 'It's a door.' Denizen Hardwick doesn't believe in magic - until he's ambushed by a monster created from shadows and sees it destroyed by a word made of sunlight.