How can I persuade readers why Uchida's book is an argument in a 500-word essay?

The most important element in writing a critical essay is its structure. You should try to present an engaging introduction, a well thought-out argument, and a well-supported conclusion. In this case, your introduction might include some general information about Uchida’s book. For instance, you may start your essay like so:
Desert Exile: The Uprooting of a Japanese American Family is a 2015 nonfictional, autobiographical novel written by Japanese American author Yoshiko Uchida. It is, in fact, a memoir of sorts, which tells the author’s own account of her life during and after the Second World War, and the hardships and imprisonment of the Japanese American people (many of whom were United States citizens) by the American Government. The novel consists of eight chapters and an epilogue, in which Uchida explains her reasons for writing the book and recaps all of her experiences.
Then, you should focus on presenting the main point of the essay. You should start by writing that Uchida’s book is an argument in itself, in the fight against discrimination and racism, as it provides detailed information about the oppression and the mistreatment of the Issei—the often-poor Japanese immigrants who came to the US to find a better life and provide for their families, and the Nisei—the children of these Japanese immigrants, who were, basically, American citizens. They rarely spoke Japanese, and their knowledge of Japanese culture and their origin was primarily gained through their parents' stories and testimonials.
The memoir explains the effects and consequences of such immoral and evil notions, and encourages the readers to fight for justice, equality and acceptance of all people, no matter their race, gender or socioeconomic status. You should also mention that Uchida’s novel is told from her point of view only, as it is a retrospective retelling of her and her family’s struggles during and after the Second World War. My advice would be to elaborate a bit more on the subject, and even use some quotes from some of the chapters that will further strengthen the point of the argument. I’d suggest chapter 3, as it explains the attack on Pearl Harbor, which was a tragic event in American history, where many Japanese Americans were questioned on their loyalty and intentions.
Finally, you should conclude your essay by stating that Uchida’s novel is, essentially, a reminder of something that we do not wish to happen again. It focuses on massive sociological and moral problems which mustn’t be ignored and left unsolved, because we must not allow history to repeat itself. I would also suggest exploring the novel’s themes and conflicts, as they might prove useful in your research.