The boy at the top of the mountain, from John Boyne

Hello, I am Japanese.
When I first read this book, I thought it was a masterpiece.
It is the best novel on the theme that “people are easily influenced by their surroundings and can become good or evil depending on the people they associate with.”
I was eager to share my thoughts on the book, so I put the name “John Boyne” into reddit and did a search, but all I saw was a lot of criticism of “the boy in the striped pajamas” (I didn’t read much of the book).
Why is that? Please elaborate.

PS: In the afterword of the Japanese translation, it says that there is a common character in the three books together with Mr. John’s “Stay Where You Are And Then Leave,” but I couldn’t find it for the life of me.
If you know more about it, please tell me who it is.

by KaldomEX