AS EVERY HARASSED commuter knows, there would seem to be no more private activity than reading. Open a novel and you leave behind the torture of the 6.20 from Paddington or Victoria for the world you share with your favourite authors. Look down the carriage and you will see fellow travellers doing the same, each enclosed in his own literary cocoon. In such cases, reading becomes a retreat, a sanctuary, a place of safety and fulfilment rarely to be found in ordinary experience. Yet, as William St Clair shows in his new book, this notion of reading as distinctively private is comparatively new. It may even be a product of the exhausting modern life from which it provides a refuge.