Making ASCII values more usable as part of hash key

Question

I'm to implement a hash function, and here is my hash function (the first draft version that is)

    public int hashCode(){
        String fixedISBN = getIsbn().toString().replace("-", "");
        fixedISBN = fixedISBN.substring(fixedISBN.length()-4, fixedISBN.length());
        int ISBN = Integer.parseInt(fixedISBN);
        int ASCII = 0;
        for (int i = 0; i < getTitle().toString().length(); i++) {
            ASCII += getTitle().toString().charAt(i);
        }
        int hashValue = (ISBN * 37 + ASCII*23);
        return hashValue;
    }

I am meant to hash books, and to do so I initially thought to use the ISBN value of a book, which serves as a wholly unique identifier for every book. Then I looked at the list of ISBNs and saw that using the entire ISBN since there isn't a lot of variation of the ISBN numbers. As such I use only the last four numbers of the ISBN since those numbers tend to be the ones that vary. I also plan to use the ASCII value of the title's chars for my hashValue, but I believe a problem arises since ASCII values can only amount to 127, which means there would be a problem if the title is short, say only 8 chars or less which would produce a maximum value 1016. If the table size is very large, say 10 007 it wouldn't produce a very even spread. Is there any way I could make ASCII values more suitable to produce a hash value of a large table