An ISBN will only be necessary if you intend to distribute or market your book. If your book is for personal use only or to give out as gifts, then you can register the book with the relevant authorities or have us to do it for you.
ISBN (International Standard Book Number) is a unique identification number that is used by libraries and booksellers the world over to order your book or get distribution information on it. Once again, you will only need an ISBN if you intend to market your book.