In general, e-books from the biggest vendors — Amazon, B&N, and Apple — are only readable using their own devices or on computers and smartphones and tablets (using their own apps). This is because they each have their own Digital Rights Management (DRM, or copy protection) systems.
NOOK allows you to read e-books from sellers that use Adobe's DRM system, which is pretty much every other e-book seller besides those three. If you're worried about transferability, you could buy your e-books from them and those will be readable on almost anything except for a Kindle. You lose the convenience of wireless shopping, though, and often the prices are higher elsewhere.
A few e-books don't have DRM and so are easily transferred to other readers. Unfortunately, B&N doesn't indicate which those are. Generally, it's by publisher. Carina, Samhain, Smashwords, O'Reilly, Microsoft, and a number of erotica publishers don't require DRM. Tor recently announced that they're going to quit using DRM. But that's a pretty limited list.
There's also the illegal (in the US) option of stripping the DRM from the e-books that you buy so that you can put them on a different device. Some people choose to do this, relying on the extreme unlikelihood of ever getting sued so long as they don't distribute the stripped e-books.
E-books are mainly useful for fiction and for narrative non-fiction like memoirs, and most people are content to read an e-book just once. Consequently, lifetime access isn't a big deal. If necessary, one can always use a computer, smartphone, or tablet running the appropriate app. Or one can simply buy an appropriate e-reader,,, prices continue to drop, and within a couple of years the price of a dedicated e-reader will probably be less than the cost of a hardcover book.
