Do you need an ISBN?
When publishing a book, your expenses can start to add up. An ISBN is one of those expenses that falls in the gray area. Is one (or two) really needed?
First off, let's talk about what an ISBN actually is. In layman's terms - it's your book's social security number. If you don't purchase one yourself and assign it to your book when it is published, then each retailer can assign their own version of a social security number, and then your book is dealing with multiple personalities and the very slight possibility of identity theft.
If you spend your free time swimming in cash, then buy an ISBN. Right now - GO. You'll actually need to buy two, one for the ebook and one for the paperback edition, and they are $125 each. You can buy them here.
But if money is tight (and $250 is painfully expensive), then you need to decide if you really need one. And you can always purchase an ISBN at a later date, so don't stress over this decision too much.
Here are the reasons you may need an ISBN:
1. You are trying to get your book into traditional brick and mortar stores. If you are campaigning and are putting serious effort into getting bookstores to stock your title, you need an ISBN for the paperback. It's the professional thing to do, and it will cause you to look less like a self-published newbie, and more like a traditional publisher.
2. You are trying to hit the New York Times bestseller list for Combined Print and eBook. If you are going after this goal (and please know it is a feat of Olympic proportions), you will need to purchase an ISBN so that all of your sales accumulate and are properly credited.
3. You want to maximize your novel's online presence. I almost hesitate in writing that line, because I'm not certain that 'maximizing your novel's online presence' has absolutely ANY impact on sales. But Google likes it when your ISBN matches across all retailers. And you need an ISBN in order for your novel to be included in Books in Print, which is a giant database that (apparently) booksellers use to make decisions about what to purchase.
For me ... I get my ISBNs at a discount from my literary agent. So it's a no-brainer for me. But if I didn't receive them at a steep discount - if I had to pay $250 for them every time I published a book ... *sigh*. I don't know that I would. I've already hit the NYT list. I'm not that concerned with being in stores or being in a catalog. And I'm painfully cheap, so I don't spend hundreds of dollars unless I can verify that I will receive a return on that investment.
This is one of those decisions that you have to make on your own. If you do decide to purchase one, you can do it here.