It’s a good idea to get one if you plan to sell your book to libraries, but you don’t have to have it printed in the book itself (so it’s OK if you do apply for it while your book is printing). All you have to do is apply for a Library of Congress (LC) Pre-Assigned Card Catalog Number (PCN). It doesn’t cost anything and it can be ordered for any book over fifty pages (genealogies and children’s books fewer than fifty pages are an exception to this rule).