XML/XQuery/count

Count nodes

File: Data.xml
<?xml version="1.0"?>
<Books>
 <Book>book 1</Book>
 <Book>book 2</Book>
 <Book>book 3</Book>
</Books>

File: Query.xquery
<library count="{count(doc("Data.xml")/Books/Book)}">{
  for $b in doc("Data.xml")/Books/Book
    return <book>{$b/text()}</book>
 }</library>

Output:
<?xml version="1.0" encoding="UTF-8"?>
<library count="3">
   <book>book 1</book>
   <book>book 2</book>
   <book>book 3</book>
</library>