could you try if this works:
$book_type ='';
$book_type_field = get_field_object('book_type');
$book_type_values = get_field('book_type');
$count = count($book_type_values);
foreach ($book_type_values as $key => $value) {
$book_type .= $book_type_field['choices'][ $value ];
if (($count - 1 - $key) >= 1){$book_type .= ', ';}
}
echo $book_type; //do this whenever you wish (this variable should contain all chosen labels, separated by a comma)
or do you get also a error with this code