Why The London Library, ‘a labyrinth of culture’, is also one of the capital’s most surprising social hotspots – just ask Tom Hanks and Helena Bonham Carter

‘The London Library is a rare beast,’ says historian Giles Milton, ‘life would be immeasurably poorer if it didn't exist’ – and taking a photograph outside its grand façade on St James’s square is a new form of social currency for book-loving American celebrities