LHP is correct. The bar credit books or coupons or whatever you want to call them are on the website for people to purchase as bon voyage gifts for friends that are sailing. You won't save any money by pre-paying for drinks in this manner. In fact it is probably a bigger hassle than its worth (you have to have the coupons with all the time, then you have to worry about the cost of the drink, etc etc... ) Don't waste your time buying them for yourself.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007