I'm going to offer two ways to do this. One is what I'd consider to be a best practice (first), and the other is the smallest modification to your code (second).
First: How I would do it:
Note: Storing data like this in tables is not ideal, DIV elements are designed for this. I'm not going to try to rewrite it though. This is just a best practice with regards to the JS.
I would suggest using the <span> tag for this, as you can style it to display exactly as if it wasn't there (it's an inline rather than block element, but it's very easy to select.
...
<a href = "http://www.google.com" target = "_blank"><img src = "example.jpg" border = "0"/></a><br /><br />
Author : Jason<br />
Book Title : Example Book<br />
Release Date : 2013.05.11<br /><br />
Description : <br /><br />
<span id='description'>Here is where I want to add some text</span><br /><br />
Hello, example....<br />
<br />
...
To replace this, you can simply do the following:
document.getElementById('description').innerHTML = "Your Text";
Second: How YOU want to do it:
This is not to say that it's impossible to replace without using the span. You would have to give your DIV an id descriptor instead of (or as well as) class (a class is not unique so there'd be no way to tell which DIV you're trying to affect.
You could read the entire HTML of the auto-thumbnail, then use a regular expression to replace a placeholder, something like this:
<div id="auto-thumbnail">
<table width = "500px" height = "auto" border = "0" align = "center">
<tbody>
<tr><td>
<a href = "http://www.google.com" target = "_blank"><img src = "example.jpg" border = "0"/></a><br /><br />
Author : Jason<br />
Book Title : Example Book<br />
Release Date : 2013.05.11<br /><br />
Description : <br /><br />
{DESCRIPTION}<br /><br /><!--Add text on this line-->
Hello, example....<br />
<br />
</td></tr>
</tbody>
</table>
</div>
What you'd do here is have a JavaScript function which gets the inner content of this DIV, then replaces the {DESCRIPTION} placeholder with the description you want.
function replaceDescription(newdescription) {
var regexp = new RegExp("{DESCRIPTION}");
document.getElementById('auto-thumbnail').innerHTML = document.getElementById('auto-thumbnail').innerHTML.replace(regexp, "Your Description");
}
This will be much slower than the first format, though, so I totally recommend using that.
<div>or<p>tags instead of<br>