If you are in the list view, instead of the book cover view, you can swipe across an entry to get an Archive button for single books. If you press and hold a book you will get a minus icon next to all the books. You can tap on the minus to get the Archive button. This is quicker, sort of, for removing multiple books.
You'll notice that it says Archive, not delete. That is because it only removes the book from the iPad. It will become available on the Archive page, incase you ever want to re-download it. Permanent removal from the Archive can only be done on the Amazon web site. Unless you really hate having that book in the list you should leave it there. It won't take up any space sitting in the Archive (on Amazon.com's site).
