How to get two JavaScript classes to talk to each other?
There are multiple ways of handling this. If you can pass the Books you want to the Shelf object, that would be probably one of the easiest methods.

//The Shelves
function Shelf (shelfNumber, shelfName) {
    this.shelfNumber = shelfNumber;
    this.shelfName = shelfName;
    this.books = new Array();
    this.addBook = function(book) {
        this.books[this.books.length] = book;
    }
    this.displayBooks = function() {
        for(i=0; i &lt; this.books.length; i++) {
            displayBooks(this.books[i]);
        }
    }
}

// then you can use it like this...

shelf100.addBook(bookA);
shelf100.addBook(bookB);
shelf100.addBook(bookC);

shelf100.displayBooks();