Negating certain words from a custom rewrite expression?

Hi. I have a WP page named Book (/book/) which displays a book in various languages. The language and chapter variables
are passed as query variables. So the URL structure looks like:

/book/english/ (This displays a list of chapters in English)
/book/english/foreword/ (This displays the Foreword of the book in English)

Here is what I have come up with:

add_action('init', 'book_init');
add_filter('rewrite_rules_array', 'book_rewrite_rules_array');
add_filter('query_vars', 'book_query_vars');

function book_init() {
	global $wp_rewrite;
	$wp_rewrite->flush_rules();
}
function book_rewrite_rules_array($rewrite_rules) {
	global $wp_rewrite;
	$custom['(book)/(.+)/(.+)$'] = 'index.php?pagename=$matches[1]&book_language=$matches[2]&book_chapter=$matches[3]';
	$custom['(book)/(.+)$'] = 'index.php?pagename=$matches[1]&book_language=$matches[2]';
	return $custom + $rewrite_rules;
}
function book_query_vars($query) {
	array_push($query, 'book_language', 'book_chapter');
	return $query;
}

Everything is working but the problem is that, the rewrite rules I have added are also catching /book/feed/ which I don’t
want. So I am looking for an expression which would negate feed from '(book)/(.+)/(.+)$' and '(book)/(.+)$'

Also I want to know, if assume the query variables supplied are invalid, which filter should I use to check this in and how
can I stop WP from continuing and instead make it send a 404 error and have it show the 404 page?