function on trigger

hi guys

I'm newbie in Psql and I'm trying to build one function  in order to
count the products for each supplier. So i'm gonna put it quite simply
though this example


Please, consider a table called books with the following fields

bookid, title, price, idPublisher



and one another table called publisher

Idpublisher, name, city, Books


Being in that last field from Publisher, called book, I gotta have the
amount of published books for each publisher.