The stump of Sarah's middle finger is a major catalyst for her trips down memory lane, which begin at Andrew's funeral. Here's one example:
I sat in the pew, massaging the stump of my finger, and found myself acknowledging for the first time that my husband had been doomed since the day we met Little Bee. (2.17)
This line serves as an intro for the series of tangled memories Sarah will have, and through which the story – including how she lost that finger (she cut it off when it was clear Andrew wasn't going to cut off his) – is revealed.
Sarah seems to be using her memories to help figure out how to deal with her present situation, and to think about who she is and who she wants to be. Her stump is a symbol of all that she's willing to sacrifice in order to do the right thing. The sense of justice Sarah develops over the course of the novel is in stark contrast to that of the hunter, the man responsible for the loss of her finger. Or is it? Are there any parallels? What do you think Sarah's finger symbolizes for Little Bee? Charlie? Lawrence? What did it symbolize for Andrew?