For Seniors: E-book Sources Besides iBookstore for Your iPhone 4S
The iPhone 4S is capable of using iBooks and other e-reader apps to read book content from other bookstores, so you can get books from sources other than iBookstore. To do so, first download another e-reader application such as Kindle from Amazon or the Barnes & Noble Nook reader from the iPhone App Store. Then use their features to search for, purchase, and download content.
The Kindle e-reader application archives any content you've already bought from the Amazon.com Kindle Store online, and that content can be placed on your Kindle Home page on the iPhone for you to read anytime you like. To delete a book from this reader, press the title with your finger, and the Delete button appears.
You can also get content from a variety of other sources: Project Gutenberg, Google, some publishers like Baen, and so on. Get the content using your computer if you like and then just add the items to Books in iTunes and sync them to your iPhone.
You can also make settings to iCloud so that books are pushed across your Apple devices or place them in an online storage service such as Dropbox and access them from there.