i would say lincoln, abraham lincoln.
the fact that it's still a part of american history, after all these years, or at least until obama puts a tax on that page in the history books, says something. everyone knows who killed him, or shot him that is.
will princess di be in the history books in 100 years? maybe in england, but doubtful in america
so the fact that the US has way, way, way more students than england means it's more "popular" and it's in more books, until obama creates a book czar that taxes pages of books above page 15.