I used up my $6 video credits and all my credits went down to $0 - instant video, software/games, and Kindle. So, unless there's another balance somewhere for books...
ETA: You guys are right. Maybe all those credits I mentioned are really one in the same, but I just added a book to my cart and it took $8 off for "No-rush shipping book credits". Not sure where you can view your balance but if you add an expensive book to your cart, you'll be able to see how many you have.