''The Timewaster Diaries'' is the third outing for Robin Cooper, the fictional alter-ego of TV producer Robert Popper ( ''Peep Show''; ''Look Around You''). In his previous two books of ''Timewaster Letters'', Cooper published the rambling and slightly surreal prank letters he sent to a range of real companies and organisations. Their often hilarious comedy sprang from the usually polite but gradually more exasperated replies sent by the unwitting victims as the correspondence progressed.