For you kind information using iBooks app you can read and download books from the included iBookstore. Simply tap a book to open it and swipe to flip through pages. iBookstore requires iPhone 4, iPhone 3GS, iPhone 3G, iPad, iPod touch 4G, iPod touch 3G, iPod touch 2G running version iOS 4.0 or later. In order to download iBookstore you will need to have an Apple ID.
You should note that the books those are published in the ePub electronic book format or documents in the PDF format can be opened by iBooks. iBooks is available worldwide exclusively through the App Store in the Books category for free of cost. You can download and get more information about iBooks from here [iTunes Link].