Depression can charge out of nowhere, dumping you headlong into despair. Or it can sneak up on you in tiny increments, like salt from an icy road that splatters the underside of a car and slowly corrodes the metal, and you may not even realize it. "I would guess that only 20 percent of people with mild depression recognize it for what it is and seek help," says Andrew Solomon, author of the new book, The Noonday Demon: An Atlas of Depression.   Find out just how much you really know about depression, and learn to recognize the myths Solomon discovered.