Previous_Post_Link – Extra character?

I suppose you could use one of the functions listed here (http://codex.wordpress.org/Function_Reference) but determining which page is first or next would depend on how you sorted the results.

By default I assume the posts are returned by PKID and can possibly be ordered by DATE or TITLE, etc.

Look at the function singature in the wp-includes/post.php to see what functions take what arguments.

This is how I would do it with my limited knowledge of WordPress:

$cur = get_the_ID(); // Get ID of currently active page?

// You could probably use another funciton to actually filter
// so that you only look for pages under category A for instance
$arr = get_all_page_ids(); // Get array of ALL pages
$arr = array_values($arr); // Only interested in PKID's

$prev = prev($arr[$cur]);
$next = next($arr[$cur]);

$pipe = false; // TRUE means we vertical pipe separator is required

if ($prev !== false) {
  $pipe = true;
  echo '<a href="">Previous Posts</a>';
}

if ($next !== false) {
  if($pipe === true){
    echo '&nbsp;|&nbsp;';
  }
  echo '<a href="">Next Posts</a>';
}

Totally un-tested but it should give you an idea as to where to start???

Hope it helps 🙂