Okay, I have read two of the books (Paddle to the Sea and Minn of the Mississippi), and I don't see the "many spiritual ideas not pertaining to Christ" that the reviewer at Christianbook.com mentioned. In Paddle, the second chapter has the Indian boy who carved the Paddle Person saying, "The time has come for you to sit on this snowbank and wait for the Sun Spirit to set you free." I see this small comment as easily explained to children as Native American culture and beliefs. Nothing else that I read seemed to need any editing or explanation.
Likewise, I found only a couple of minor issues in the book Minn of the Mississippi. In the first chapter, the fifth paragraph refers to a "land of ancient waters" and goes on to describe periods of earth's formation as having occured for millions of years. If you are a creationist who believes in a young earth, you may want to skip this paragraph or just edit it a bit as you read. It seems as though the chapter would read just fine if you decided to skip this paragraph entirely. Another mention is made of turtles having been alive on this planet "at least some one hundred seventy-five million years!" along with a picture of dinosaurs. And there is a chapter entitled "Mother Earth," but that phrase is not used again in the chapter. I know (and my children know) that God is our Father and we do not consider the earth to be our mother, so I plan to just not bother reading the title of that chapter. Or, if I did, it would be easily shrugged off.
All in all, I don't see any major reason to leave these books out of our curriculum, and we're pretty conservative Christians. However, I do see a LOT of reasons to read these books....they are sweet, well-written stories, and they are wonderful in exploring particular regions of North America. I wouldn't want to leave them out. They are wonderful books!