$this->primaryKey = 'isbn';
return $this->hasMany('Other', 'isbn');

class Book {
    public function isbn() {
        // instead of hasMany
        return ISBN::where('isbn', $this->isbn);
    }
}

public function getKeyName(){
        return $yourKeyName;
    }

return $this->hasMany('Comment', 'foreign_key', 'local_key');