Iterator NPE in java

I'm having an issue with multiple NullPointerExceptions, I'm getting one for my if statements within iterators and I can't figure out why

one of the problem blocks:

public int removeAllBooks(String author, String title){
Iterator<Book> itr=library.iterator();
int i=0;
while(itr.hasNext()){
        Book book=itr.next();
        if(b.getAuthor().equals(author)&&(b.getTitle().equals(title))){
            itr.remove();
            i++;
        }
        if(i>0){
            return i;
        }
     }
return 0;  

}

The NPE error points to my if-statement line for some reason.

Thanks.