How do I use this for buying e-books?

Hey, you may dont need to spend a lot of many on buying ebook in iTunes or Kindle ,Kobo, Adobe, you just need to pay the rent money and then downloaded on your devices, after that, you can remove the DRM and then share to other devices. Check this guide out Can Read Kindle Books on My Android Phone

Hi do what the previous suggest but to download books from amazon you will have to download the kindle app from the iTunes store it is free to download it and just click on it to open and go to the kindle store I do it quite a lot . I have loads of books on it . You will need an Amazon account though but the books are a lot cheaper than buying them in a book shop

Purchasing ebooks through the iBooks Store is very similar to buying any other media via Apple's iTunes Store. One key difference, though, is how you access the store. Rather than through iTunes or a dedicated app like the iTunes Store app on the iPad and iPhone, in this case, you access it through the iBooks app.
In the top left corner of the iBooks app is a button that reads "Store". To access the iBooks Store and begin shopping for iPhone- and iPad-compatible ebooks, tap that button.

Once you enter the iBooks Store, the browsing and searching experience is very similar to using the iTunes Store. In this case, you can search for the author or title you want in the top-right corner search box.
To browse books, either tap the icons on the main page of the store or tap the "Categories" menu item in the top left corner. This drops down a menu with all of the iBooks store's categories in it. Tap the category you're looking for to see books in that genre.

In the iBooks Store, when you tap a book title to get more details (synopsis, reviews, etc.) a window pops up and the background grays out a bit.
In this window, you can download a free sample of the book to your iBooks library by tapping the "Get Sample" button. This is the equivalent of iTunes' 30-second song preview, which helps you get a sense of whether you want to purchase the book.
When you decide to purchase a book, tap the price on the left side of the screen next to the book cover. The button will turn green and read "Buy Book". Tap it again to buy the book.
To close the pop-up window and not buy the book, simply tap anywhere outside the window.

You buy at the iBooks Store using the same iTunes account and password as you do at the iTunes Store or App Store. When buying a book, you'll be prompted to enter your password to pay for the ebook before it will begin downloading.

Once you've entered your iTunes account password, the eBook will download to your iPad. How long this takes will depend on the book (its length, how many images it has, etc.).
When the book is done downloading, it will appear as a title on the bookshelves in the iBooks app. Tap it to begin reading.