When do I need to get an ISBN? How do I get one?
R.R. Bowker Company was named exclusive provider of International Standard Book Numbers (ISBNs) in the United States in 1968 (an awful year, otherwise). Today, the company (now just plain Bowker) is part of ProQuest and still pumping out ISBNs as well as standard bar codes for publisher/bookseller inventory management. And, it offers a range of other publishing services and more. A good company. Bowker does license others to help sell ISBNs, such as Publisher Services. But watch out for nefarious ISBN distributor wannabes.
Your books need ISBNs. Every version, edition, and format needs its own ISBN, which can be purchased one-at-a-time (very expensive) or in lots on a steep discount curve. E-Books don’t require ISBNs. If you are publishing using Amazon CreateSpace, you can get your one-off ISBN from them at no cost at all. Sweet deal.