Can a Nook Book & Kindle eBook Be Downloaded on Either Device?
The electronic reading market hasn't seen the type of winner-takes-all battle that happened with VHS and Betamax or Blu-ray and HD-DVD. However, you may still decide to change your brand of e-book reader or even use multiple models. Amazon's Kindle and Barnes & Noble's Nook are among the highest profile options; in some cases you can read books from one company on the other company's device, but you may face limitations in other cases. Your options will depend on whether you want to download directly to the device through an Internet connection or get the book on your device via another method, known as sideloading.
1 Downloading to Kindle
You can only download books to the Kindle if they are already in the correct format for Kindle (such as MOBI or the dedicated AZW). For the most part, this means using the official Kindle Store, reached from a direct link on Kindle readers and the built-in app on Kindle Fire tablets. When using the Web browser on the tablets or the experimental Web browser on some Kindle models, you can sometimes download directly from some sites that offer MOBI books and have them appear in your device's library, though this can be inconsistent. You can't download onto a Kindle directly from the Nook Store, but you can install and run the Nook app on a Kindle Fire.
2 Sideloading: Kindle
You can add books to a Kindle device by emailing it to a dedicated address for your device (which is detailed on the "Manage My Kindle" page on the Amazon site when you are logged in to your account) or through Windows Explorer when you connect the Kindle to your PC via a USB cable. As with downloads, the file must be in a suitable format. Books from the Nook Store are usually in EPUB format, so you'll have to convert them with independent software first.
3 Downloading: Nook
If you have a dedicated Nook e-book reader (as opposed to a tablet), you cannot access the Kindle store. If you have a tablet, you can access the Google Play store and in turn install the Kindle App. This lets you download books you have previously bought from the Kindle Store on your Nook tablet. Note that you read any such books through the Kindle app rather than via the main library and reading tool on the tablet.
4 Sideloading: Nook
It is possible to add books to your Nook device via USB transfer rather than a download. These books will appear in the Documents folder rather than your main book library. It is technically possible to convert Kindle books to the EPUB format required for reading on the Nook using independent software. However, most Kindle books carry digital rights management protection; the removal of this protection violates Amazon's Terms of Service and is a legally gray area, so doing so may be a criminal or civil breach depending on your location.