You can not delete it.
If you go to the Purchased section in iTunes on the computer you can choose to hide the book by clicking on the (x) that appears near the top left of the icon. If you hide something by accident you can go into your account setting to unhide it.
Apple works on the (usually good) assumption that if you bought it, you deserve to be able to download it again, and shouldn't lose it because of some accidental click.
If for some reason you absolutely must have this book expunged from your history forever, you'll need to contact Apple iTunes support and request it. I don't know if they can or will do it, but that's the only route that might work.
Last edited by Justinian; 11-30-2012 at 12:00 PM.
If the book has been deleted from your iPad, it is not on the iPad. If it is showing as "purchased" - that is a reflection of what is on Apple's servers. It is not on your iPad, so it's not taking up space on it.
What Marilyn said.
If the book shows with a cloud symbol, or says Download or Install where the price usually is, then it's not on the iPad. It's a link for you to download the book again if you like. This is also true of other other iStore content like apps, music, TV shows, and movies.