A.S.
I can't think of any cute wording, but I just recently hosted a shower in which we used board books as balloon weights. The new mom and dad got to take them home, and it allowed me to display a few small bunches of balloons around the room.
As for the look for your invitation, a "bookworm" theme would be cute. I'm sure the local craft stores have something in their scrapbooking departments that would work.
Good luck and have fun!
A.