Order by & Group by in XQuery

example.xqy

let $books :=
  <books>
    <book class="fiction">Book of Strange New Things</book>
    <book class="nonfiction">Programming Scala</book>
    <book class="fiction">Absurdistan</book>
    <book class="nonfiction">Art of R Programming</book>
    <book class="fiction">I, Robot</book>
  </books>
for $book in $books/book
let $title := $book/text()
let $class := $book/@class
order by $title
group by $class
return ($class, $title)