You should be using a class on the editable elements instead of an ID, like so:
<a id="username" class="editable editable-click" data-pk="1" href="#">Book Name 1</a>
<br>
<a class="username" data-pk="2" href="#">Book Name 2</a>
<br>
<a class="username" data-pk="3" href="#">Book Name 3</a>
<br>
<a class="username" data-pk="4" href="#">Book Name 4</a>
Which means your JS should now be:
$('.username').editable({
type: 'text',
url: 'post.php',
title: 'Enter username'
});