The Best Dog in the World by Miriam Moss
Henry doesn't like dogs. Especially his grandad's dog, Harvey, who lives next door and barks at the postman. Harvey doesn't like little boys much either. When Henry is playing in the garden, Harvey runs and hides.
But when Henry and Harvey find themselves marooned in a barn during a thunderstorm, they discover that dogs and little boys aren't so bad after all. In fact, they can even become best friends.