Of course for the limited sample you could be tempted to inline anything
as an XPath 3.1 expression.

  <xsl:template match="/">
    <xsl:sequence
      select="map {
               'books' : array {
                  books/book ! map {
                    'title' : title!string(),
                    'authors' : array { author/string() }
                  }
                }
              }"/>
  </xsl:template>