I have been getting lots of "PHP Notice: Undefined variable: wpbook_lite_message wpbook-lite.php on line 547" messages in my debug .log
I think they are triggered when another plugin updates a post?
In any case I eliminated the message by replacing the update_post_meta call with
if ( isset($wpbook_lite_message) ) update_post_meta($post_id, 'wpbook_lite_message', $wpbook_lite_message);
I am not sure if this is the correct action, or if deleting the post meta when there isn't a message would be correct?