This book is geared towards 2nd graders that read well. My 8 year old loved it. He has never been to Yosemite.
I have been to Yosemite though. So the fact that the author had folks gathering around campfires for food, and even more, has Ethan hiding his candy in his sleeping bag really bothered me. At least back in April 2013, Yosemite was very particular about food due to bears. All the food, or even something that might have food has to be put in the bear locker at night. Looking at the park website, they still say to be careful with food because of bears, so the fact that this was never even mentioned made the author seem less knowledgeable about Yosemite.
My one other complaint about the book was the "I'm my own Grandpa" conversation. I know there was a song written with the same title (according to wiki back in 1947), but really?
The treasure hunt and the fire were actual plot points, which made it better than some of the books in the series. Oh, and I did appreciate the map in the front of the book!