Harry potter，popular with most young people，can serve as reading material.

形容词短语做定语， 因为过长， 避免头重脚轻，所以后置，如果单独是popular则需放在前面就ok了

哈利波特，最流行的年轻人，可以作为阅读材料

不是做定语的呀，在这个句子里是做插入语成分的~可以这么用~
你查查插入语的用法就好了~

哈利波特，受大多数年轻人欢迎，可以用作阅读材料。作定语，which is popular with....

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.

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.
Then every thing is clear.
The grammar book definitely won't tell you exactly how to create a sentence like this,even a little bit misleading and confusing especially when your English is not so good.The best way to increase either your confidence in doing the multiple choice or the reading and writing skills is through reading.Reading a large amount of materials,magazines,thrillers,rommance