The question perhaps is not how forgotten Joseph Patrick Kennedy, Jr. is -- but how well known he was to start with although the firstborn namsake declared at an earlier age: ""Someday I'll be President."" He died equally prematurely on a mission in World War II (it was supposed to have been his last and it was) in an attempt to dump explosives, on the V-1 sites across the Channel. Joe Kennedy was as a young man the most devastatingly attractive of them all with a ""magic smile,"" a great flair for the ladies, and an easily triggered temper. He did well (not first string) on the football playing fields of Harvard and also graduated cum laude. All of this plus his easy socializing both here and on the Continent is recontructed via the family, the Navy files, and interviews, and Mr. Searls, until now a novelist, gives it a high gloss finish. With the exception of his brother Jack's privately printed As We Remember Joe, there is no other portrait.