Publish content to main domain and subdomain

You have two options, both of which require custom code.

1) Use hook_form_alter() to auto-select the domains on the node form. This could be a little tricky, and it would allow users to change the settings.

2) Use hook_domainrecords() to always assign book_type nodes to the primary domain, which sounds like what you want. That would look something like so:

// This assumes you have a module named custom.module.
function custom_domainrecords(&$grants, $node) {
  global $_domain;
  // We only care about the book_type node.
  if ($node->type != 'book_type') {
    return;
  }
  $primary_set = FALSE;
  foreach ($grants as $key => $grant) {
    // Remove the domain_site grant.
    if ($grant['realm'] == 'domain_site') {
      unset($grants[$key]);
    }
    // Remove domains we don't want.
    else if ($grant['gid'] != $_domain['domain_id']) {
      unset($grants[$key]);
    }
    // Check that the primary domain is set.
    if ($grant['gid'] == 0 && $grant['realm'] == 'domain_id') {
      $primary_set = TRUE;
    }
  }
  // Make sure the primary domain is selected.
  if (!$primary_set) {
    $grants[] = array(
      'realm' => 'domain_id',
      'gid' => 0,
      'grant_view' => TRUE,
      'grant_update' => TRUE,
      'grant_delete' => TRUE,
      'priority' => 0,
    );
  }
}

You might have to do a combination of both. Note that your hook_form_alter() implementation must fire after domain_form_alter().

See API.php in the download for details on these hooks.