diff -u -r1.15.2.4 book_access.module
--- book_access.module	21 Nov 2009 19:39:12 -0000	1.15.2.4
+++ book_access.module	23 Nov 2009 00:56:44 -0000
@@ -132,6 +132,9 @@
 /**
  * Implementation of hook_node_access_explain().
  *
+ * hook_node_access_explain() is defined in devel_node_access.module, which
+ * will help you and your users to figure out how node access works and what
+ * permissions are currently granted.
  */
 function book_access_node_access_explain($row) {
   static $roles = NULL;
@@ -140,15 +143,20 @@
     if (!isset($roles)) {
       $roles = user_roles();
     }
+
     if (isset($roles[$row->gid])) {
-      //return array($roles[$row->gid]);
-      return t(
-        'grants for users of role %role',
-        array('%role' => $roles[$row->gid])
+      return array(
+        t('grants for users of role: %role',
+          array('%role' => $roles[$row->gid])
+        )
       );
     }
 
-    return array('(unknown gid)');
+    return array(
+      t('Unknown group id %gid: It is recommended to rebuild node access',
+        array('%gid' => $row->gid)
+      )
+    );
   }
 }