modifying SELECT to query by meta-key and meta-value

Hi

I have the following query code that I need to modify to also search by a specific meta_key and meta_value pair in a specific post-type. I have tried changing the code but I am getting empty results.

This is the original code:

$letters = $wpdb->get_col(
"SELECT DISTINCT LEFT(post_title,1) AS first_letter FROM $wpdb->posts
WHERE post_type = '$post_type' AND post_status = 'publish'
ORDER BY first_letter ASC"
);

I tried to change it to:

$post_type = 'directory-listing';
$meta_key = 'services';
$meta_value = 'Book Storage';

$letters = $wpdb->get_col(
"SELECT DISTINCT LEFT($wpdb->posts.ID_title,1) AS first_letter FROM $wpdb->posts, $wpdb->postmeta
WHERE $wpdb->posts.ID = $wpdb->postmeta.post_id AND
  $wpdb->posts.post_status = 'publish' AND
  $wpdb->posts.post_type = '$post_type' AND
  $wpdb->postmeta.meta_key = '$meta_key' AND
  meta_value = '$meta_value'
ORDER BY first_letter ASC"
);

As I said I get an empty result.

Can anyone spot my issue?

Thanks in advance for your help!