Query that joins two documents

For each book found at both bn.com and amazon.com, list the title of the book and its price from each source.

<books-with-prices>
   FOR $b IN document("http://www.bn.com/bib.xml")//book,
     $a IN document("http://www.amazon.com/reviews.xml")//entry
   WHERE $b/title = $a/title
   RETURN
    <book-with-prices>
     $b/title,
     <price-amazon> $a/price/text() </price-amazon>,
     <price-bn> $b/price/text() </price-bn>
    </book-with-prices>
</books-with-prices>

Adapted from XML Query Use Cases