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

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