How to insert book cover in the sidebar template?

You mean like a “What I’m reading” thing ?
If so…

Resize the book image so it will fit. Upload it somewhere.

open sidebar.php
move to where you want the book to appear, then type

<li id=”books”><h2>Reading</h2>

<ul>
<li>
<img src=”http://www.marketingbyann.com/images/book.jog&#8221; alt=”Reading” />
</li>
</ul>