declare function local:toc($book-or-section)
{
    for $section in 
         $book-or-section/section
    return
      <section>
         {$section/@id,  $section/title , local:toc($section) }                 
      </section>
};


<toc>
   {
     for $s in doc("book.xml")/book return local:toc($s)   
   
   }
   
</toc>