This thread has gotten out of hand; but as long as we're going for Perlishly obscure, here's something silly...

#!/usr/bin/perl -wp
use strict;
use vars '$letter';

s/^(?!\Q$letter\E).*//is;

BEGIN {
    @ARGV = 'books.txt';
    print 'Letter: ';
    chomp($letter = <STDIN>);
}
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