To be fair, there arent a lot of American authors of prominence these days.
The one who cries out for a Nobel is Tom Wolfe.
But he wont get one of course, politics.
A genre author of genuinely high literary merit, and still living, whose works are probably outstanding examples of US letters, is the science fiction author Gene Wolfe - yeah, theres something about all those wolves.
Of course, that's because Archie wrote his books for him.