Invisible ink or secret writing (SW, as it is known in the trade) has probably been used since not long after mankind learned to communicate in writing. Often viewed as the poor relation of its sophisticated sibling cryptography, it nonetheless thrived in the 20th century. Perhaps, with the recent decline of written communication, it will die out. Who, after all, sends letters in these days of texts and tweets and whatever? From the spy’s point of view (spies are overwhelmingly the users of SW) there’s always safety in numbers and to be one of the dwindling band of letter-writers might attract attention. But, as Kristie Macrakis indicates in this comprehensive history, SW is already quietly mutating into digital image-hiding.