Have you ever thought why you are able to compare physical books only and not e-books? Why search engines don't search e-books online malls or other websites that sell exclusively e-books.
I won't mind if a search engine crawls every website that sells:
* PLR articles packs
* MRR products
I was directed to leatherbound.me that searches for e-books only. If you click on the search results individually, you can see their prices on different outlets that include Kindle, Nook and iBook Store.
Look at the screenshot below after I clicked on one of the e-books shown in the search result. Also note the Tweet button at the end of the page for quick social media sharing.
Click on the image for larger views.