Problem syncing book downloaded using iBooks Using iTunes I tried to sync my iPad with my PC after downloading an epub book using iBooks on my iPad. 

However I get an error stating that my PC is not authorized to sync the book to my iPad and after getting this error message i noticed that the book that I downloaded is no longer available in my iBook library on the iPad, which is weird because the book was already on my iPad in the first place.

How do I resolve this issue?

I'm a relatively new user on the iPad so I apologise if this may seem like a silly question.
Authorize your Computer

I'm fairly certain that your issue is that your computer is not authorized with the Apple ID used to buy the iBook.  Therefore when you synced, the book was not transferred and then the iPad was synced. (Book basically deleted, sorry)

The fix is fairly simple, I believe you should be able to redownload your iBook for free, per Apple's new rules with previous purchases.  Then authorize your computer. From the Store menu in the iTunes menu bar, choose "Authorize This Computer".  Type in your account info, and then sync your computer/iPad.

Hope that works! :)