Why it is a remarkable book, particularly pertaining to the court case, and not the love story of Hatsue & Ishmael?
When creating a thesis, try to avoid broad statements like "remarkable book;" create a statement that needs proving. Why is it a remarkable book? What makes the court case stand out?
For example: The trial of Kabuo in Snow Falling on Cedars is an accurate depiction of racial prejudice against Japanese-Americans after World War II.
(What makes the previous statement a thesis is that it follows the thesis formula: Topic + Opinion = Thesis Statement)
Assuming you chose this thesis, you may want to focus on how the police officers immediately jumped to the idea of a Japanese person because of the slash on Carl's head; the idea that the ship may have caused the cut never crossed their minds, it immediately went to a person. You could discuss how ludicrous it would be to integrate ancient Japanese-style fighting on a pitch-black boat in the middle of the ocean. You could also discuss how Kabuo was so reluctant to tell anyone anything because of the understood prejudice, and how that reaction created even more racism. Discuss the difference between Nels and the prosecution -- discuss how these two different lawyers are characterized so differently; why did Kabuo get the "old, crazy-eyed lawyer." Though we know how good Nels is, you may discuss how the prejudice of the court even affects how they view the lawyer, and thus affects the outcome of the trial.