This is a good question to consider before reading: we want to know what kind of book this is, or how we should categorize it in relation to other books. In other words, what's the genre?
One answer is that The Cay is a work of historical fiction. That means it's a made-up story, but it takes place in a historically interesting time. We should understand that the main characters, Timothy and Phillip, are created by the author and aren't real people we'd find in a history textbook. But at the same time, we should understand that their adventure will reveal to us something about the real situation going on in the world during the time the story takes place. That is, the story says something about what life was like in 1942, during World War II, especially regarding the conflicts among the countries, the state of unrest throughout the world, and the tense relationships between people of different races.
Another way to describe The Cay's genre is "juvenile fiction." That means it's a story written specifically for young people to read and enjoy; knowing this, we should expect the tale to be relatively short and simple. We should expect the general message and theme of the story to be positive and uplifting, even if there's tragedy in the plot itself. These expectations hold true for The Cay: it carries a strong message about friendship and the need to overcome racial tensions. We also see a great deal of hope and optimism coming from the characters as they deal with being stranded on an island.
Finally, you could even categorize The Cay with other award-winning novels. Though it hasn't won a Newbery medal, it has gathered an impressive list of awards, including the New York Times Best Book of the Year and the School Library Journal Best Book of the Year. These awards indicate that the book should be taken seriously, because it has an important message to share with its readership and because the way it's written has been recognized as masterful.