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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.