Commerce module in not drupal commerce I have a website, built as a non Drupal commerce website. I want to add functionality to my website so customers can buy books, and add checkout page and cart.
I mean add commerce functionality in a not Drupal commerce site.

Can I do that?
Yes that is possible. Download the modules from: https://www.drupal.org/project/commerce

You can not install the install profile (https://www.drupal.org/project/commerce_kickstart) on top of an existing Drupal website.