Instruct: Classify the given review into positive or negative sentiment.
Query: A pretty deep book for a children's book.

I've read almost all of the Hardy Boys books and this is my favorite. It separates the two main characters; Frank and Joe, and shows how alone they can be without each other. At a couple of points the book goes very deep as Frank pictures his life without Joe and his brother "dying without even knowing who he was". It shows a side of Frank that has never been seen before; it also shows to what lengths each boy will go to save the other. I liked the book a lot. Hardy Boys Casefiles#80 is another extremely psychological book in which Frank and Joe get separated and deal with things alone. However, I felt #80 was a little too emotional as I almost began to cry at a couple of points. I think both books are worth reading, but in order to understand them you have to read Casefiles #1 first; #11 and #80 both refer back to seens in this book without explaining them (it's assumed you know what happened in #1). If you're a Hardy Boys fan you must read this book