Q: Preordered book is not able to be purchased.

I preordered a book off of iBooks and I was sent an e-mail telling me to claim the book. I currently have enough money in my iTunes account to make the transaction but when I click "OK" to download the book I am left with Insufficient the message "Insufficient Remaining Store Credit." If I return to iBooks and try to download the book manually from there, the option to purchase is not there and when I click "Read" I am approached with "This item is no longer available." Any help received is appreciated, thank you.