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If not, tap the Sign In button and enter your email address and password.  Use the tabs at the top of the screen to show books not already on your device and tap the Download button (a cloud with an arrow in it) to download a book.

Summary:
Tap the iBooks icon on your device's Home screen to launch the iBooks app. Tap the Store button at the top of the interface. Scroll down to the bottom of the screen and check that you are signed in to your iTunes account. Tap the Purchased button at the bottom of the screen. All of your previous purchases are now listed.