One of the reasons I keep a working diary while researching a book is because it is almost impossible otherwise to remember the moment when you discovered something crucial about your subject. By the time I am ready to hand in the manuscript, I feel that I have always known everything about this person, when in truth it was a slow process of discovery.

But on the subject of what mattered most to Ethel Rosenberg—which, after completing my biography of one half of the Soviet-spy couple, I know was her role as a mother—my diary was of little help.