--- modules/book/book.module.orig	2009-02-25 12:47:37.000000000 +0100
+++ modules/book/book.module	2010-05-17 00:01:10.000000000 +0200
@@ -214,6 +214,37 @@ function book_block($op = 'list', $delta
           // There should only be one element at the top level.
           $data = array_shift($tree);
           $block['subject'] = theme('book_title_link', $data['link']);
+          /* With many pages on the same depth one level below the index page for the book
+          the bookmenu becomes very long. This code was added to create a "context sensitive"
+          menu when browsing a book: an x number of pages above the current one and the same number
+          of pages below the current one is shown in the book menu.
+          At the index page and the first (x + 1) pages only the first (2*x + 1) pages are shown. */
+          if (isset($data['below'])) {
+            $no_shown_above_below = 10;
+            $totaal = count($data['below']);
+            if ($totaal > (2 * $no_shown_above_below + 1)) {
+              $nummer_in_array = 0;
+              $on_index_page = 1;
+              // we need to know where we are in the menu array
+              foreach($data['below'] as $key => $subdata) {
+              $nummer_in_array++;
+               if ($subdata['link']['in_active_trail'] == 1) {
+                $on_index_page--;
+                break; // active page found
+               }
+              } // end foreach($data['below'] as $key => $subdata)
+              if (($nummer_in_array < ($no_shown_above_below + 1) && $on_index_page == 0) | $on_index_page == 1) {
+              // early page or on index page
+                $data['below'] = array_slice($data['below'], 0, (2 * $no_shown_above_below + 1));
+              } elseif ($totaal - $nummer_in_array < $no_shown_above_below && $on_index_page == 0) {
+              // close to the end of the number of pages
+                $data['below'] = array_slice($data['below'], ($totaal - (2 * $no_shown_above_below + 1)), (2 * $no_shown_above_below + 1));
+              } else {
+                $data['below'] = array_slice($data['below'], ($nummer_in_array - $no_shown_above_below - 1), (2 * $no_shown_above_below + 1));
+              }
+            } // end if ($totaal > (2 * $no_shown_above_below + 1))
+          } // end if (isset($data['below']))
+
           $block['content'] = ($data['below']) ? menu_tree_output($data['below']) : '';
         }
       }