Is this just going to be an image or an animation? If it's a still image, you could use an image plane for the sky, moon and background landscape. For the book, yes, polygon modelling sounds good. For the interior scene, I would create and render that part separately, and composite it together after in photoshop or after effects, depending on whether it's animated. For the girl, just create flat polygons in the proper shape, so it doesn't have any depth. Hope that gets you in the right direction.