ES: Opening the 5th book component from the end

This is how I would do it:

#target framemaker

var comps = [];
var book = app.ActiveBook;
var bookComp = book.FirstComponentInBook;
while (bookComp.ObjectValid ()) {
    // put the components into the array in reverse order.
    comps.unshift (bookComp);
    bookComp = bookComp.NextBookComponentInDFSOrder;
}

alert (comps[4].Name);

I cannot see any alternative to iterating through all the book components, storing the component ids as you go..

I'd use a buffer big enough to hold only 5 component ids. Store each successive component id you find in location (counter modulo 5) - so it's effectively a circular buffer. When you reach the last component in the book, the component you want will be the next entry in the circular buffer. This method saves memory, and handles a book of any size.

Whether or not you use the above method, ensure you gracefully handle a book with fewer than 5 components. In the above case, I'd intiialize the buffer to nulls, and check for a non-null value at the end.