> What's stopping the book from containing that information?

Because in the thought experiment, the translation function (which is what you mean by the book) is f(Chinese) -> Chinese. If you don't have knowledge of Chinese, it won't help you.

We can also play a little game. I’ve invented a language called Kejdoke.

Me: Jeheixis ownc j’Lm hys bjækf¿

You: Kloften¡’by

You should be able to tell me what this exchange was about. Since any reasonable definition of understanding for a language would be to do this, and you have the same knowledge of Kejdoke as the person in the thought experiment has about Chinese. If you can't tell me what this exchange was about, the person in the thought experiment could not understand either.

This just alters the thought experiment. What the thought experiment offers is a book of opaque rules, just instructions on how to transform symbols into more symbols without any concern for their meaning. What you're offering is a translation dictionary, what does it mean when one encounters X symbol. You're giving him a book on learning Chinese.