THE HOSTAGE By Order of the President, the first novel in W.E.B. Griffin's crackling new Presidential Agent series, won immediate acclaim from critics and fans alike.   Charley Castillo works with the Department of Homeland Security, but more and more he is the man to whom the president turns when he needs an investigation done discreetly. And no situation demands discretion more than the one before them now.  An American diplomat's wife is kidnapped in Argentina, and her husband murdered before her eyes. Her children will be next, she is warned, if she doesn't tell them where her brother is-a brother, as it turns out, who may know quite a bit about the burgeoning UN/Iraq oil-for-food scandal. There is an awful lot of money flying around, and an awful lot of hands reaching out to grab it-and some of those hands don't mind shedding as much blood as it takes.  Brimming with rich characters, strong action, and cutting-edge drama, this is Griffin writing at the height of his powers.