Currently, in Drupal 5x, top-level book page parents have no breadcrumb whatsoever, not even a link to the main or home page. The following code adds a breadcrumb that includes the title of the page as well as a link home.

Find the code in your page.tpl.php file that looks something like this:

<?php if($breadcrumb) { ?> 
  <div id="breadcrumb">
    <div class="wrapper">
      <?php print $breadcrumb; ?>
    </div>
  </div>
<?php } ?>

Replace that code with this:

<?php if($breadcrumb) { ?>  //if the node already has a breadcrumb, print the normal one
  <div id="breadcrumb">
    <div class="wrapper">
      <?php print $breadcrumb; ?>
    </div>
  </div>
  <?php } 
  elseif (!$is_front) { ?> //checks to see if the page is not the front page, and if it isn't, prints the site name as a link to home, a chevron, and then the title of the current page
    <div id="breadcrumb">
    <div class="wrapper">
      <a href="<?php print base_path(); ?>"><?php print $site_name; ?></a> » <?php print $title ?>
    </div>
  </div>
<?php } ?>