S.S.
This REALLY bothers me too. People need to have better manners. You have to plan for the number of people to any party. I usually call the ones that have not responded 2-3 days in advance of the party and let them know that they have been invited to a party and I am SO VERY SORRY that their invitation must have gotten lost because I had not heard from them. Some "fess" up and some don't. Anyway, at least you have a contact and they will hopefully let you know then if they're coming. Alternately, you don't have to make up anything and just ask them if they are coming to the party or not because you want to plan for the right number of people. I like the book of etiquette idea . . . Have fun entertaining!