M is a dazzling book which I imagine will spawn as many imitators as Longitude. It fits into no known genre: it is written with all the bravura of a novel but is not fiction; it is based on fact, but the facts are too sparse or too questionable to constitute a full biography. No matter. What M does is to introduce a completely new way of looking at paintings in their historical context. Normally historical context means some huge overview (‘the origins of the Renaissance’, ‘the death of Mannerism’), but M evokes the day-by-day context of a painter’s working career.