I understand completely. I prefer to purchase my iBooks through gift cards. If you go into the AppleStore directly to check your funds it will take a bit but will give you credit. Then purchase your book from there, not in iBooks.
I came across several issues since the recent updates and was trying to contact Apple to bring attention to them.
1) books I preordered on iBooks no longer automatically download or debit from my gift card.
2) gift card balances do not immediately credit to iBooks
3) if I try to preorder a book on iBooks it wants to take it out of my credit card
4) I have to log in to Apple if I want to read a book in my collection previously purchased.
THESE ISSUES NEEDS TO BE CORRECTED
I want my ability to purchase books in advance so they download automatically and I want my funds correctly available. Not to much to ask