Linda Chapman 
My Secret Unicorn: Starlight Surprise [EPUB ebook] 

Lauren’s little brother, Max, has been having bad dreams about ghosts. Their friends at school all think that a treehouse in the woods nearby is haunted and won’t go near. Lauren and Twilight, her secret unicorn, decide to investigate. Lauren is sure there’s no such thing as ghosts – they’re the stuff of fairy stories, aren’t they? Just like elves and fairies and… unicorns? The spooky noises from the wood make Lauren and Twilight very nervous but they face their fears together and eventually they reveal the 'ghosts’ to be selfish boys from school who tricked everyone into leaving the treehouse for them alone to use as a den. Lauren and Twilight turn the tables on the boys with a magical surprise of their own…