Follow the viewing instructions of each download site.  

When you  click buy, you will be sent to either the iBookstore site, the site, or the Amazon (Kindle) site, where secure transactions of the books will take place; and if you want to return to the site later, you can click the back button on the tools bar, once or several times. After buying a book, revisit this website, from time to time, and check for improvements on the content of the book you purchased: Click for updates