A Unicorn, A Dinosaur, and a Shark Were Riding a Bicycle

Nobody wants to read a book about unicorns, dinosaurs and sharks just relaxing. That's just boring! According to the narrator of this book, a unicorn, a dinosaur and a shark were riding a bicycle. But they say that's not true - they just want to take a load off for once. They're tired of being told what to do by the bossy storyteller! And who is the reader of the book supposed to believe anyway - the book's narrator or its characters?