Not without removing the DRM (encryption).
For B&N, see Buying eBooks from B&N
For Amazon, you would use Kindle for PC to download the ebook and unswindle.pyw to remove the DRM. Note that most Kindle books are AZW (MOBI), which is what unswindle works on, but a few are TOPAZ. For the most part it isn't practical to move TOPAZ ebooks to other reading devices yet.