I think these etiquette rules are enriched here on the bump. IRL, I doubt suggesting a book instead of a card would be "sideeyed" or seen a host "dictating" what a guest should buy. Obviously, the guest of honor gets baby related gifts for this occasion. By the reasoning of this board, suggesting that the gifts be baby related is "dictating". In reality, it's nonsense. I've never been to a shower where there was the book instead of the card thing going on, however I doubt anybody I know would think twice about it. You've been reading the bump too much if you find yourself panicking about a psuedo faux pas.
I went to a shower this month where it was "suggested" I get a book instead of a card. I DEFINITELY side-eyed it. It made me really uncomfortable, as I didn't see the stupid little insert until after I had purchased the baby gift with the amount I had budgeted. But I felt obligated, so I went out and bought a book. And, books absolutely do not cost the same as cards, unless you want to go to the dollar bin at Target and get a book that will quickly fall apart, which, by the way, isn't a "favorite children's classic" like the invites dictate you get. I ended up getting a $5 Kohl's Cares for Kids book. But you know what? Money is tight right now, and I absolutely hated that I went over-budget on this stupid thing, and wish that they wouldn't have asked me to do that. If MTB wants books, she should register for them and people will get the hint, and buy baby a book if they want to. And if the hostess/MTB absolutely loves the 'sweet notes to baby in a book' idea, then the hostess can buy a few books for guests to write in.