If we update metadata then add the calibre uuid.

book.php

@@ -313,6 +313,7 @@ class Book extends Base {
             $epub->Language ($this->getLanguages ());
             $epub->Description ($this->getComment (false));
             $epub->Subjects ($this->getTagsName ());
+            $epub->Calibre ($this->uuid);
             $se = $this->getSerie ();
             if (!is_null ($se)) {
                 $epub->Serie ($se->name);

php-epub-meta/epub.php

@@ -259,6 +259,15 @@ class EPub {
     public function Amazon($amazon=false){
         return $this->getset('dc:identifier',$amazon,'opf:scheme','AMAZON');
     }
+    
+    /**
+     * Set or get the Calibre UUID of the book
+     *
+     * @param string $uuid
+     */
+    public function Calibre($uuid=false){
+        return $this->getset('dc:identifier',$uuid,'opf:scheme','calibre');
+    }
 
     /**
      * Set or get the Serie of the book