Hi Graham,
Thanks for visiting Apple Support Communities.
If you've purchased books from the iBooks Store but don't want them to show up on your Mac, use these steps to hide them:
Use iTunes on a Mac or PC to hide or unhide your purchases.
- Open iTunes on your Mac or PC.
- From the Store menu, choose Store > Sign In, and then enter your Apple ID and password.
- Go to the iTunes Store.
- Click Purchased from the Quick Links section on the right side of the iTunes Store.
- Choose a content type by clicking on Music, TV Shows, Apps, or Books from top.
- Hover your cursor over the item you want to hide.
- You will see an "X" either to the right or left top corner of the item.
- Click the "X" to hide that item. A confirmation dialog will appear once you click the "X."
See the full article with screenshots here:
iTunes Store: Hiding and unhiding purchases
http://support.apple.com/kb/HT4919
Regards,
Jeremy