Fetches the menu link for the next page of the book.
$book_link: A fully loaded menu link that is part of the book hierarchy.
A fully loaded menu link for the page after the one represented in $book_link.
function book_next($book_link) {
$flat = book_get_flat_menu($book_link);
// Assigning the array to $flat resets the array pointer for use with each().
do {
list($key, $curr) = each($flat);
} while ($key && $key != $book_link['mlid']);
if ($key == $book_link['mlid']) {
return current($flat);
}
}