IIRC, your CC number is not transmitted anywhere. It is used by the reader software as the key to let you into the eBook.
To unlock the book in order to read it, I have to enter my credit card number???
Into the Ipod???
and have that number sent over a network???
It isn't sent over the network. The e-book is encrypted with a password that is automatically created by scrambling the name and credit card number that was used to buy the e-book.
In order to decrypt the e-book for display, the app needs the name and credit card number so that it can scramble them into the appropriate password.
You should only have to enter that information one time. The iPod app should remember the password after that. (Note: it doesn't remember the credit card number, just the password).