Sunnylands, the impeccably haute-moderne Shangri-la completed in Rancho Mirage, California, in 1966 by the late billionaire publisher, philanthropist, and power broker Walter Annenberg and his wife, Lee, was a haven for presidents and monarchs, stars and tycoons: the history-makers of the late 20th century. Now, as it becomes a nonprofit center—“the Camp David of the West,” in Annenberg’s vision—Bob Colacello, a former guest, recalls Sunnylands’ private magic. By Bob Colacello ,Photographs by Todd Eberle.....