Just copy the code into a php custom block. Make sure you replace $book_top_page with the book id you wish to show.

Drupal 6.x

(This was added by user littlelloyd for Drupal 6.)

<div class="whole-book-menu">
<?php
$book_top_page = 1;
$levels_deep = 3;
$emulate_book_block = true;

if (!function_exists('book_struct_recurse')){
function book_struct_recurse($nid, $levels_deep, $children, $current_lineage = array(), $emulate_book_block = true) {
$struct = '';
if ($children[$nid] && ($levels_deep > 0 || ($emulate_book_block && in_array($nid, $current_lineage)))) {
$struct = '<ul>';
      foreach ($children[$nid] as $key => $node) {
      if ($tree = book_struct_recurse($node->nid, $levels_deep - 1, $children, $current_lineage, $emulate_book_block)) {
      $struct .= '<li>';
      $struct .= l($node->title, 'node/'. $node->nid);
      $struct .= $tree;
      $struct .= '</li>';
      }
      else {
      if ($children[$node->nid]){
      $struct .= '<li>'. l($node->title, 'node/'. $node->nid) .'</li>';
      }
      else {
      $struct .= '<li>'. l($node->title, 'node/'. $node->nid) .'</li>';
      }
      }
      }
      $struct .= '</ul>';
return $struct;
}
}
}

$current_lineage = array();

$result = db_query(db_rewrite_sql('SELECT n.nid, n.title, n2.nid parent, ml.weight
FROM {node} n
INNER JOIN {book} b ON n.nid = b.nid
INNER JOIN {menu_links} ml ON b.mlid = ml.mlid
INNER JOIN {book} b2 on b2.mlid = ml.plid
INNER JOIN {node} n2 on b2.nid = n2.nid
WHERE n.status =1
ORDER BY ml.weight, n.title'));

while ($node = db_fetch_object($result)) {
if (!$children[$node->parent]) {
$children[$node->parent] = array();
}
array_push($children[$node->parent], $node);

/*  This function is broken, and for my purposes, not needed **********
if (arg(0) == 'node' && is_numeric(arg(1)) && arg(1) == $node->nid) {
  $_temp = book_location($node);
  foreach ($_temp as $key => $val){
    $current_lineage[] = $val->nid;
  }
  $current_lineage[] = arg(1);
}
*/
}

echo book_struct_recurse($book_top_page, $levels_deep, $children, $current_lineage, $emulate_book_block);

?></div>

Drupal 5.x

<div class="whole-book-menu">

<?php
$book_top_page = 63;
$levels_deep = 2;
$emulate_book_block = true;

if (!function_exists('book_struct_recurse')){
function book_struct_recurse($nid, $levels_deep, $children, $current_lineage = array(), $emulate_book_block = true) {
$struct = '';
if ($children[$nid] && ($levels_deep > 0 || ($emulate_book_block && in_array($nid, $current_lineage)))) {
$struct = '<ul>';
      foreach ($children[$nid] as $key => $node) {
      if ($tree = book_struct_recurse($node->nid, $levels_deep - 1, $children, $current_lineage, $emulate_book_block)) {
      $struct .= '<li>';
      $struct .= l($node->title, 'node/'. $node->nid);
      $struct .= $tree;
      $struct .= '</li>';
      }
      else {
      if ($children[$node->nid]){
      $struct .= '<li>'. l($node->title, 'node/'. $node->nid) .'</li>';
      }
      else {
      $struct .= '<li>'. l($node->title, 'node/'. $node->nid) .'</li>';
      }
      }
      }
      $struct .= '</ul>';
return $struct;
}
}
}

$current_lineage = array();

// use this version for Drupal 4.6
// $result = db_query(db_rewrite_sql('SELECT n.nid, n.title, b.parent, b.weight FROM {node} n INNER JOIN {book} b ON n.nid = b.nid WHERE n.status = 1 ORDER BY b.weight, n.title'));

// use this version for Drupal 4.7 and Drupal 5
$result = db_query(db_rewrite_sql('SELECT n.nid, n.title, b.parent, b.weight FROM {node} n INNER JOIN {book} b ON n.nid = b.nid AND n.vid = b.vid WHERE n.status = 1 ORDER BY b.weight, n.title'));

while ($node = db_fetch_object($result)) {
if (!$children[$node->parent]) {
$children[$node->parent] = array();
}
array_push($children[$node->parent], $node);

if (arg(0) == 'node' && is_numeric(arg(1)) && arg(1) == $node->nid) {
$_temp = book_location($node);
foreach ($_temp as $key => $val){
$current_lineage[] = $val->nid;
}
$current_lineage[] = arg(1);
}
}

echo book_struct_recurse($book_top_page, $levels_deep, $children, $current_lineage, $emulate_book_block);

?></div>