We were new to the A to Z Mysteries series, but excited when we got this book as a gift, since it involves panning for gold in Montana (which we did on vacation a couple years ago). I read the book aloud for my boys (age 8-1/2 and 12), and all three of us were bored with it for the first half of the book.
The gold nugget wasn't mentioned until chapter 4 and didn't disappear until chapter 7. Therefore only chapters 8 to 11 (the final chapter) actually involved solving the mystery. The rest was just filler about dude ranch activities and hanging out with friends. We're used to Scooby and other children's mystery books where the mystery is established much earlier in the book.
On the positive side, the book has an illustration in almost every chapter and a manageable amount of text per page/chapter, which is definitely a plus for kids who are just starting to read chapter books on their own.