We did this at my shower and it was wonderful. I got a TON of nice books and all came with nice gifts. The cards I got where thrown in the trash can along with the wrapping paper.
Like people said if you don't like the idea of the book instead of a card then don't buy a book and quit compalining about it.
Can I make a suggestion? Forget books at the shower - you will probably get some anyway - and while books are always important, it's not like babies read just yet.
For my son's first birthday, I asked people to forgo gifts entirely and bring a book instead. That way, we didn't get a zillion more toys that I would've had to find a place for AND we built up his library at a good time (when he was more interested in listening to me read to him).
Even though I was, in fact, telling people what to bring, I think most of them appreciated the fact that they didn't have to pick out another toy for another kid, etc. When you have kids, all of a sudden you go to 100 birthday parties a year. So, a book was an easy gift for them to buy, and it costed about the same as a toy would have. A few people still brought toys, and that was totally fine as well.
Just a thought.
When phrased properly, it's a suggestion. Just like including a card with where you are registered is a suggestion of where to shop for the new baby. I'm surprised at how irritated people are about this.
That being said, my baby shower is a library theme. And my invites have a poem suggesting a book instead of a card. And I don't really care if someone gets annoyed that it was proposed that they bring a book. If you don't like it, don't do it. And if it bothers you that much, don't attend the shower.
I can't even fathom having a problem with the whole book instead of card suggestion. I've been to two showers this year where this was on the invitation. For one, I brought "The Giving Tree" - one of my favorite books as a child. I did not write in it, and actually included the return slip- assuming there could be another copy given. I attached a gift tag to the gift. At the next shower, I simply forgot and did not even add one to the gift. Who cares???
I honestly wonder how people have time to think about this and make it something to be offended by. There are SO many things people do that are offensive - this is really not one of them. If you want books, it's a good way to get the point across.