Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/modules/book/book.js

@@ -0,0 +1,32 @@
+/**
+ * @file
+ * Javascript behaviors for the Book module.
+ */
+
+(function ($) {
+
+  "use strict";
+
+  /**
+   * @type {Drupal~behavior}
+   */
+  Drupal.behaviors.bookDetailsSummaries = {
+    attach: function (context) {
+      $(context).find('.book-outline-form').drupalSetSummary(function (context) {
+        var $select = $(context).find('.book-title-select');
+        var val = $select.val();
+
+        if (val === '0') {
+          return Drupal.t('Not in book');
+        }
+        else if (val === 'new') {
+          return Drupal.t('New book');
+        }
+        else {
+          return Drupal.checkPlain($select.find(':selected').text());
+        }
+      });
+    }
+  };
+
+})(jQuery);