How to get alphabetic listing x other posts, based on first letter post?

For the solution you can use if statement
as below
put it in your sidebar, it will highlight the current post title.

<ul>
<?php
$IDOutsideLoop = $post->ID;
$args = array( 'posts_per_page' => -1, 'orderby'=> 'title', 'order' => 'ASC' );
$myposts = get_posts( $args );
foreach($myposts as $post) :
?>
<li <?php if(is_single() && $IDOutsideLoop == $post->ID) print 'style="font-weight:bold";' ?>><a href="https://wordpress.stackexchange.com/questions/99310/<?php the_permalink(); ?>"><?php the_title(); ?></a></li>
<?php endforeach; ?>
</ul>

Even wordpress provides for highlighting the current pages here,
http://codex.wordpress.org/Dynamic_Menu_Highlighting
it doesn’t include to hightlight the specific post’s title at sidebar.

I don’t know if you are looking for static navigation , the below link might help you .
http://blog.huidesign.com/automatically-highlight-current-page-in-navigation-with-css-jquery/