declare function local:section-summary($book-or-section as element()*)
  as element()*
{
  for $section in $book-or-section
  return
    <section>
       { $section/@* }
       { $section/title }       
       <figcount>         
         { count($section/figure) }
       </figcount>                
       { local:section-summary($section/section) }                      
    </section>
};

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