Who Owns Letters?

There's a big stink going on in Italy right now over the publication of some love letters written by Italo Calvino to his mistress Elsa de' Giorgi. The letters were sold by De' Giorgi Fondo Manoscritti di Pavia, a manuscript archive. The archive promised to wait 25 years before publishing any of them. Well the 25 years are up and they have begun publishing them much to the unhappiness of Calvino's widow and daughter. They are claiming that De' Giorgi didn't have the right to sell them and the archive can't publish them without her permission since she is literary executor of Calvino's estate. An interesting conundrum. Calvino is clearly the author of the letters, but he wrote them to De' Giorgi and so shouldn't she have the right to do with them as she pleases? It's no wonder so many authors want their correspondence and notes burned upon their death.