If you think it's important for a child to get to know an author who writes or illustrates books, it is equally important for a student to own a book created by an author whom they've met. You can have sales handled by a local book store or dealer. No matter how you buy the books, you should be able to negotiate a discount which you can either pass on to the students or keep to defray the cost of the author visit. Book sales are maximized when you send out a notice about two weeks before the author visit listing the author's books, along with the retail price and tax, so the students can order books in advance. Book sales are further maximized when a book table is set up at the assembly so teachers and parents who haven't yet ordered can choose what they'd like to buy.
Book sales can benefit the schools in several ways.
Second, think how much fun it will be for students to read and show off a book that was signed for them by the author. Think how much fun it will be for teachers to read from and talk about a book by an author who visited your school. The benefits of an author visit should continue long after the assembly or classroom visit is over. If you don't take advantage of an author visit in the days and weeks after the visit, you're missing a lot of potential for learning. Making books or posters celebrating your favorite stories or poems created by the author.