Trout Fishing in America - Richard Brautigan (whimsical and beautifully innocent encapsulation of the "hippie" experience)

A Separate Peace - John Knowles (a "Catcher In The Rye"-type book, but more nuanced and beautifully written)

The Great Gatsby - F. Scott Fitzgerald (if one points to an example of THE great American novel, this book is in that category)

Johnny Got His Gun - Dalton Trumbo (one of the greatest - if not the greatest - anti-war books ever written)

Lolita - Vladimir Nabokov (Nabokov turned a taboo relationship between an older man and a young girl into a work of art)

Tropic of Cancer/Capricorn, The Rosy Crucifixion - Henry Miller (Miller makes beautiful writing look effortless)