It all started with a pastry. In 1920, the young Joan Miró (1893-1983) arrived in Paris carrying an ensaimada from Pablo Picasso's (1881-1973) mother in Barcelona. The Catalonian capital meant everything to both of them. Miró was born there, Picasso spent his formative years there (1895- 1904), and in the end, both men gave large portions of their life’s work back to it. It was an unlikely introduction to one of the great friendships in modern art, one that would last more than fifty years. The two artists could hardly have been more different. Picasso was restless, prolific, and larger...