Yes, Maybe: ISBN stands for "International Standard Book Number." It's a ten-digit number, soon to be thirteen-digit, that is a unique identifier for your book. It's important because your book may have the same or similiar title to other books in the system. Without the ISBN, customers may wind up with the wrong books. Therefore, you need one if you are planning to sell your book to resellers (bookstores, distributors, wholesalers, Amazon, etc.). You don't need one if you know you are only going to sell your book directly--to family, to workshop participants, online from your own website, etc.
Generally, it's a good idea to get an ISBN. If you're book is published by a traditional publisher, the ISBN will be provided by the publisher. As a "self-publisher," you will either do without, get your own set of ten ISBNs, or use an ISBN from a vendor who provides some publishing services but is not a true publisher (an on-demand printer, for example). The Publishing Pro, LLC, does not like the practice of providing an ISBN when a company is not truly publishing a book. This practice creates confusion, adding to the impression of the author that she has been published by an outside publisher when she has actually been sold a set of editorial, graphic, or printing services. While we have nothing against providing such services--that's what we do, after all--we do not like our clients to leave us with the impression that we are their publisher when we aren't. The impression won't help them be successful. Therefore, we send clients direct to the ISBN Agency where they can use their credit cards to order a set of ten ISBNs (standard delivery is about $250, including the registration fee). Not all countries make you order ten ISBNs at a time, but the U.S. agency does, probably because they would lose money selling you one number at a time. On the other hand, we like our clients to be thinking about doing their second and third books, in which case they won't have to re-order their numbers. The ISBN is used to create the bar-code on the back of the book, something you can order through the ISBN agency. However, we provide bar codes and our clients can bypass this service. The Publishing Pro, LLC.