Since you tagged jQuery to this question....
You mess with 4 languages...
One is PHP on which we have no idea what you intend to do with.
I suppose you want to send an id to server-side...
How will it be handled? and what is the expected answer?
(out of scope)
But the 3 others should be managed like this:
CSS:
#mybutton{
  margin-top:5px !important;
}
HTML:
<button id="mybutton" class="btn btn-warning btn-lg" data-book-id="<?php echo $row['issue_book_id'];?>">
  <span class="glyphicon glyphicon-pencil" aria-hidden="true"></span> Send Msg
</button>
jQuery:
$("#mybutton").on("click",function(){
  $(this).attr('disabled',true);
  $.ajax({
    url: "send.php",
    type: "POST",
    data: "id="+$(this).data("book-id"),    // the id sent in POST.
    cache: false,
    success: function(html) {
      // Success message
      console.log("Ajax worked! Now What do I do with it!?!");
      console.log("Ho!... Here is the html: \n\n" + html);
    },
    error: function(jqXHR,textStatus,errorThrown) {
      console.log("Error: "+errorThrown);
    }
  });
});