Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Philosophical
We could have chosen thoughtful to describe the overall tone of the book, but when you have a monk spitting out serious head-scratchers like "To find your true identity… within the will of Tze-Yo-Tzuh… that is the highest of all freedoms" (4.64), philosophical definitely makes more sense.
Even the end of the book, with its focus on Jin's modern-day troubles, has—at its core—a major philosophical nugget. Right before Monkey leaves Jin, he tells him,
"You know, Jin, I would have saved myself from five hundred years' imprisonment beneath a mountain of rock had I only realized how good it is to be a monkey." (9.82)
All those words sound normal and make sense, but what does Monkey really mean? Does he mean that he's happy he's not a human, like Jin, because humans have it rougher? Or is he hinting to Jin that the answer to Jin's own question—"'So what am I supposed to do now?'" (9.79)—lies in appreciating his Chinese American self?
Honestly, we can make arguments for either interpretation of Monkey's statement. And that's kind of the point, isn't it? Philosophical statements don't necessarily have one interpretation; their purpose is to make you think more deeply about your way of being. Which is pretty much what Yang gives us: a book that leaves us with the feeling that we ought to think deeply about how we treat ourselves and others.