There aren't many books out there that use a vegetable in a symbolic fashion, but if authors were keener to express the inner lives of their characters in veggie fashion then I think the onion might be the most popular option.
Onions are deep. Onions have layers. They can be spring, red or pickled. They have tough skins and inner hearts. They make you cry.
In Bel Canto, guests at a party given for a Japanese businessman are held hostage. At first, they think of nothing but their possible deaths. But, as Ann Patchett points out, it's difficult to keep up that level of worry for too long, so as the days pass the fear gives way to other emotions. The terrorists start to become people to the hostages, and vice versa. For the first time it's noticed that two of the terrorists are women. And issues such as food begin to become important.
Raw ingredients are shipped in to enable the hostages to cook, but the terrorist Generals will not allow them to use knives. So the two women terrorists are told to help with the preparation.
Thibault, the Frenchman in charge of the cooking by virtue of being French, asks Beatriz, one of the terrorists, to chop the onions. At one point while she chops he picks up another knife, forgetting that he is not meant to touch them. Beatriz immediately trains her gun on him.
"He isn't supposed to hold the knife," Beatriz said in Spanish. "The General told us that. Doesn't anybody listen?" She kept her gun aimed, her heavy eyebrows pointed down. Her eyes were starting to water from the fumes of the onions, and soon there were tears washing over her cheeks, which everyone misunderstood.
The hostages think she is crying for some other reason, an emotional reason, and so her control is lost and she has become human to them. It soon becomes obvious that she's not going to ever shoot one of them, and the lines between victim and aggressor are lost.
Onions have a lot to answer for, huh?