@Andre Quinonez It is very easy to see the html source code of a web page. Just right click on the page and choose "View Page Source" or something similar. You can also view the source code of a specific element of the page by right clicking on it and choosing "Inspect".

I just did what you said and I saw that there are indeed some hidden books in the source code. The way they are hidden is the code got commented out. That means you will see something like this

<!--<div class="book_box">
                some html code in here
 </div>-->
(code between <!-- --> does not show up in the browser)

Hope that helps.