In the book,Nickel and Dimed by Barbara Ehrenreich where she tries to live on a minimum wage job, I would classify the tone as caustic if you define caustic as a biting or a stinging tone.  When she works as a Wal-Mart sales clerk and watches her fellow employees wait for the $7.99 blouse to drop in price to $3.99 before they can afford to buy it, she sees this as a stinging indictment of how the disparity between rich and poor keeps growing wider.  Her tone is biting when discussing how the poor are invisible and ignored by the world with money.  When she is a waitress, she sees the hidden world of those without  proper documents to be in this country and how they are taken advantage of by their employers.  Again, I see her tone as caustic rather than ironic.  I don't really see irony, or the opposite of what she expects anywhere in the book.  Because she is trying to get the word out about how difficult it is to live on minimum wages and that benefits are denied to many of the workers in America, I don't hear the bitterness of a sardonic tone either because that would turn off the very people she is trying to reach.