Hello there, Summerrys.
The following Knowledge Base article reviews how to access the store from iBooks:
An Internet connection is required. On iOS devices, iBooks includes the iBooks Store. Tap the iBooks icon and your bookshelf appears. In the upper corner, tap the Store button and the bookshelf flips around to reveal the iBooks Store. From the iBooks Store, you can browse books by title, author, or genre. You can also view featured books as well as books available on the New York Times bestseller lists (United States only). Tap on a book to see more details and download a free sample. When you buy a book, it will automatically appear on your iBooks bookshelf.
If you are using the iOS 7 app, you may want to try some of the recommended steps in the following Knowledge Base article for troubleshooting the application:
If an app doesn't respond to your input, or doesn't perform as expected, do the following, testing after each step:
Learn more about troubleshooting apps.
Thanks for reaching out to Apple Support Communities.