I'm pretty close with everyone coming and frankly IMO it's their fault for assuming they need to get me a gift AND a book...I've always thought it was strange you give a gift and a card, so I guess that's why I have the mindset of it being their fault for assuming the invite insinuates bringing both.
People are busy not everyone wants to make their way to the place you registered for one gift, then to the dollar store for another gift. Put it anyway you want but generally people give a gift then they put a card on it to write a msg, and so the reciever knows who it is from. If someone reccomendeds that I give a book instead of a card then I would assume they meant present as well. If any of your close friends assume the same thing it is your or your hostest's fault, not theirs. Cards are not gifts, they are cards, two seperate thing. You can make a gift a cards by giving a gift card, but even then it is not the same thing as a card. If you put money in the card, the money is the gift, and the card is the card. If it is close, and invites are out, don't stress about it. Also you know your friends, and family so if they are cool with it, that is great. For the sake of the general debate on books instead of cards I think most people would think whoever is having the shower is requesting two gifts.