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If it's on your bookshelf, you just check the box by the book and delete it. If it's a WL book you have to wait until the request is accepted and then choose "Cannot Mail For Other Reasons" if there's an RC or "Cannot Mail" if there isn't.
If it's a WL book and it's about to time out on a member-put your account on vacation. When it times out the book will go on your bookshelf on hold with the rest of your books. Then delete it before coming off vacation hold.