Is it possible to COMPLETELY delete a Book from my purchased books?
This has since moved from iTunes to within the iBooks app, but the procedure is exactly the same:


Open iBooks
Click iBooks Store in top-left corner
Click Purchased
Click the x in the top-left corner of the book you want to hide


And to unhide:


Open iBooks
Click iBooks Store in top-left corner
Click Account
Click Manage next to Hidden Purchases
Click Unhide below book in question