American novels can be controversial for several reasons. One is the exploration of dark and disturbing themes. For instance, 'The Road' by Cormac McCarthy with its post - apocalyptic and often bleak view of humanity. It makes readers uncomfortable as it shows a world without hope. Another reason is political commentary. Novels like 'Animal Farm' by George Orwell (while not strictly American but had a big impact in the US) that criticize political systems can be controversial as they can be seen as subversive. Also, novels that break taboos regarding religion or social hierarchies, such as 'The Scarlet Letter' by Nathaniel Hawthorne which deals with adultery in a Puritan society, are often controversial.