In my two experiences with the Magic Tree House series, I've not been super impressed. When it comes to these companion books though, I have been impressed both times. These are non-fiction books that go along with a certain story in the Magic Tree House series, for example this one goes with Day of the Dragon King. It basically talks about the history and culture of China.
The book basically gives a historical/cultural overview of China. It starts with where China is and what it is like geographically, and then goes into the ancient history of China and how the first dynasties were established. It works its way to modern day China and talks about various aspects of their culture like family life, education, and celebrations. It's a very brief overview of all of these things and it's focus is for kids, but even adults can learn a few things here, it's just not quite as detailed as some of those books would be.
The one interesting thing to note with this book is that it really avoids any political aspects of today's China. The only real mention of government today is how the Chinese government only really wants families to have one child. This isn't a dig at the book, it's just an interesting omission. I would have thought it would have been good to mention how the dynasties of old are no longer in place today, but that China has a different governmental structure.
I really enjoyed reading this book. It is a children's history book, so know what it is, but it still provides some good information and does it in a way that is very accessible. I may not be a big fan of the main series of Magic Tree House books, but I am a fan of these non-fiction companions.