I have found a way around it...
I have been able to buy a gift certificate using my micropay. I created a new account and redeemed the gift certificate. Bought one of the books with this. Paid for the other.
I have then gift certificated the new balance back to my original account.
I wasn't looking for any freebies and the amount of 'free' micropay that I have received means I can justify paying for one of the books twice. Such a great scheme that I don't feel bad about having paid more than I should - from my bonuses I've been able to buy 3 books!
Great site
Thanks for the advice though