Mezrich is making a cottage industry out of documenting the misbehavior of Ivy League snots: His encore to 2002's ''Bringing Down the House'' follows real Princetonians who played the Japanese markets for millions in the 1990s. At least, I'm pretty sure they're real Princetonians. You'll be damned to figure out what's true in ''Ugly Americans,'' a puzzling piece of ''nonfiction'' featuring the most bizarre author's note in recent memory: ''While this is a true story, many of the names are fictitious.... I have used the real names of historical figures.... Otherwise, no character in the book is meant to refer specifically to a real-life person.'' If that nonsensical ass-covering doesn't make your head explode, then the astonishingly unreliable narrator might. Ultimately, ''Americans'' is pretty entertaining, mostly because Mezrich's audacity matches that of his ''subjects.''