<a href = "search.php?action=filter&filled=1&whichtype=links&titlecondition=start&titlesearch=A" >A</a>
For the letter B, just change titlesearch=A">A to titlesearch=B">B. To restrict to the current category, tack on &incatid={CATID}.
Remember that custom templates can be applied to any page, so you can make a special search result template and use it by tacking &custom=yes&TID=specialtemplate onto the search URL.
For an automatic list of all 26 English letters:
<?php
$num = ord('A');
for($x=0; $x<26; $x++)
{
$char = chr($num);
echo '<a href="search.php?action=filter&filled=1&whichtype=links&titlecondition=start&titlesearch='.$char.'">'.$char.'</a><br >';
$num++;
}
?>
$num = ord('A');
for($x=0; $x<26; $x++)
{
$char = chr($num);
echo '<a href="search.php?action=filter&filled=1&whichtype=links&titlecondition=start&titlesearch='.$char.'">'.$char.'</a><br >';
$num++;
}
?>