Betting on the wrong technology can really cost you money. If you had bought many books through the proprietary Nook system, you might lose them eventually. Farhad Manjoo (@fmanjoo) writes in the New York Times a good quick overview article on how to avoid this situation. He's recommending specific technology and media solutions, which can draw accusations of bias. I think he avoids that with good explanations, and you do have to buy something at the end of the day.
Bottom line - he suggests a four step approach: Buy Apple devices (they are simple and work well), Google services (mail, calendar, maps, browser, G+ to backup photos), Amazon media (you can look at Kindle stuff everywhere), and Cloud Connectors like Dropbox and Evernote (you can access all your data and notes everywhere - their business model depends on it);