Thanks to everybody for their advise and suggestions!
I think that Nathan and I have come to an understanding and he's agreed to drop the subject of this book for now. One of the things that makes me concerned is that his reading level is 3.0 - 5.9. This is a pretty wide range and while I know he can read a book at that 5.9 level, it doesn't necessarily mean that he can grasp a good understanding of what he's reading or that the content is appropriate for a 9 year old.
Nathan's teacher introduced me to a website where I could actually read the book online (www.funbrain.com/journal). I think we'll just stick to some of the biographies that we have on hand for now and see how we do!
Thank you all again!!