well,if you consider Harry Potter as our hero Harry's name,of course you won't understand.But the "Harry Potter" here refer to the JK Rowling's Harry Potter series,a book's title....
你可能是高中语法没有学扎实吧?形容词的看家本领就是作定语来修饰中心词啊!这句话可翻译成:很受年轻人欢迎的《哈利波特》可作为阅读材料。
这句话如果变成定语从句也是可以的:Harry Potter, who is popular with most young people, can serve as reading material....