A warning about the Nook. Barnes and Noble are having sales problems with the Nook and its e-book store. Most experts are predicting its demise (although there is a slight chance M$ may buy it out). If that does happen, the nook e-books would be unreadable after the you device died. Also, Nook's DRM is tied to the credit card number used to purchase it. Once the card expires, you would have to keep track of its number to access your books.
Although I personally do not like the Kindle e-readers and it's highly unlikely I will ever own one, it is still the most popular brand, with the Paperwhite currently in the lead. Amazon's Kindle bookstore is the largest one readily available. Despite my dislike of Kindles and Amazon's own little walled garden, they are still what I recommend to most people because of the store's wide selection of books and the Kindles' ease of use. Kindle e-book readers and e-books are pretty much plug and play when used together. Plus, there are apps that will allow one to read their Kindle books on their PCs, android devices (phones and tablets), and iOS devices (iPhones, iPads, and Macs).
Whatever you choose, also keep in mind most e-book reader vendors tie their e-books to their own readers with DRM (Digital Rights Management), an especially insidious form of copy protection that can be used to, besides prevent copying, lock e-books to one brand of e-reader, cause e-books to expire after a certain amount of time (used most often with library e-books), and limit sales to a certain part of the world. If the e-book reader and book vendor goes belly up or decides to change format (both have happened), you will lose access to your e-books. That is one reason why I recommend the Kindles and their books. Amazon is not likely to go under anytime in our lifetimes. They have had a format change but they were wise enough to keep their e-readers backwards compatible so no one lost any books.
Kindles and Kindle books use a proprietary form of the MOBI e-book format. They can also read non-proprietary MOBI books as long they aren't DRM infected (they exist but they are scarce and are usually the classics, which also can be found for free). Other than Kindle books, the most popular (and superior) e-book format is e-PUB, which Kindle can't read. Most free e-books are in this format. By using calibre, one can convert e-Pubs to MOBI so they can be read on Kindles.
A word (or more) about calibre. Calibre (btw, it's properly spelled with a lower case c and is pronounced the same as caliber, per the developer) is the best e-book management program by far and has the added bonus of being free. With calibre, one can list their books, sort them, load e-book readers, search by title, author, genre, etc. Calibre has its own e-book reader although it will not read any DRM infested books. Calibre can also convert e-books from one format to another as long as they are not DRM infested. One can use calibre to manage Kindle books but can't read them.
What bothers many users about calibre, especially new users, is calibre copies one's e-books into its own "database". This rattles people who like to organize their e-books in their own folder/filename structure. However, the way calibre organizes (manages is a more accurate term) e-books is vastly superior to the archaic folder/filename hierarchy. One thing (of many) calibre can do is send e-books from its own structure to whatever structure you want (and even change the filenames) should you ever decide to leave calibre. More information about calibre can be found here
. Calibre also has its own forum here
in the Mobile Read Forums
(an excellent place to learn more a bout e-books and e-readers in general).