Add new items to a multivalue field instead of override existing values
You need to loop through all articles, saving all countries in an temp array. When the loop finished save the countries in the book node.

For example:

$countries = [];
// Loop through loaded articles.
foreach ($articles as $article) {

    // Get the "Countries" Values.
    $countries = array_merge($countries, $article-&gt;get('field_article_countries')-&gt;getValue());
}
// Set the 'countries' values to the node "book".
$book-&gt;set('field_book_countries', $countries);
// Save Book node.
$book--&gt;save();