Having researched the matter, I now know you have to have an ISBN for every format your book is available in. And Kindle is different from Nook is different from PDF is different from print-on-demand and so on. So here I am, and I want to keep all viable distribution options open. I also know that it's more economical to purchase ISBNs in bulk. Moreover, I have other work in the pipeline I hope to publish some day. In my position, many ISBNs would you purchase? Asking it another way, what formats/platforms (if that's the right term) should I look to providing ISBNs for? Kindle, yes, and POD, and what after that?