how XQUERY is actually used?

(: file: titles.xqy :)
<table>
<tr><th>title</th><th>author</th></tr>
{
let $books-doc := doc("books.xml")
let $authors-doc := doc("authors.xml")
for $b in $books-doc//book,
    $a in $authors-doc//author
where $a/@id = $b/authorid
return
<tr>
    <td>{$b/title/text()}</td>
    <td>{$a/text()}</td>
</tr>
}

(: file: titles.xqy :)
<table>
<tr><th>title</th><th>author</th></tr>
{
let $books-doc := doc("books.xml")
let $authors-doc := doc("authors.xml")
for $b in $books-doc//book,
    $a in $authors-doc//author
where $a/@id = $b/authorid
return
<tr>
    <td>{$b/title/text()}</td>
    <td>{$a/text()}</td>
</tr>
}

(: file: titles.xqy :)
<table>
<tr><th>title</th><th>author</th></tr>
{
let $books-doc := doc("books.xml")
let $authors-doc := doc("authors.xml")
for $b in $books-doc//book,
    $a in $authors-doc//author
where $a/@id = $b/authorid
return
<tr>
    <td>{$b/title/text()}</td>
    <td>{$a/text()}</td>
</tr>
}

(: file: titles.xqy :)
<table>
<tr><th>title</th><th>author</th></tr>
{
let $books-doc := doc("books.xml")
let $authors-doc := doc("authors.xml")
for $b in $books-doc//book,
    $a in $authors-doc//author
where $a/@id = $b/authorid
return
<tr>
    <td>{$b/title/text()}</td>
    <td>{$a/text()}</td>
</tr>
}

<table>
<tr><td>Title<td><td>Author<td></tr>
{
	let $authordoc := fn:doc("author.xml")
	for $book in fn:doc("book.xml")/books/book
	return
		<tr>
				<td>{ $book/title }</td>
				<td>{ $authordoc/authors/author/[@id eq $book/authorid] }</td>
		</tr>
}
</table>