The general school of thought is that it's not proper etiquette to dictate how guests spend their money. Any specific request for books/diapers/etc. on top of the expectation to bring a gift to a shower is grabby and may put off your guests.
If you really want to fill the baby library or give her something specific, then perhaps you can orchestrate that gift and arrange it as a shower activity? For instance, have some favorite books at the shower and ask guests to sign pages? Or if you want to make a quilt for the baby have the squares cut and ask shower guests to fill a square with their wishes for the baby?
Ultimately, the selection and value of a shower gift should be up to the gift giver alone.
Yeah, don't ask guests to bring specific extra gifts. Let them spend their own money on what they want.