function onReadFactory( id, clbk ) {
		return onRead;
		/**
		* Callback invoked upon reading a file.
		*
		* @private
		* @param {(Error|null)} error - error object
		* @param {(Buffer|string)} data - file content
		* @returns {void}
		*/
		function onRead( error, data ) {
			if ( error ) {
				return done( error );
			}
			clbk( id, data.toString() );
		}
	}
Returns a callback to be invoked upon reading a file.

@private
@param {string} id - file id
@param {Callback} clbk - callback
@returns {void}