code: function validateConfig(bookJson) {
      var v = new jsonschema.Validator();
      var result = v.validate(bookJson, schema, {
          propertyName: 'config'
      });
  
      // Throw error
      if (result.errors.length > 0) {
          throw new error.ConfigurationError(new Error(result.errors[0].stack));
      }
  
      // Insert default values
      var defaults = jsonSchemaDefaults(schema);
      return mergeDefaults(bookJson, defaults);
  }
docs: Validate a book.json content
  And return a mix with the default value
  
  @param {Object} bookJson
  @return {Object}