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.

An Internet connection is required. On iOS devices such as the iPad, the iBooks App 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.

Source:
iBooks Frequently Asked Questions FAQ about iBooks and the iBooks Store

Hope that helps!

uncheck the sync books box in itunes. Then try to add them manually, if this does not work try the sync again after you have uncheck the box and applied the setting.

Have you downloaded the latest update of iOS and iBooks? iOS will be in iTunes when you connect your device, iBooks will be in the app store. Apple often releases updates which deal with bugs like this.
Hope that helped!

This worked for me: Went into SETTINGS > Wi-Fi > Selected my Network (which my iPod said I was already connected to) > Selected "Forget this Network" (which disconnects you from your network) > Re-selected my network and under password, I entered the 'Security Key' from my Verizon FIOS Tech Wizard info and the device accessed the app store successfully.