B&N calls it "sideloading."
If you casn download an eBook to your computer,
and if it is in a format that the Nook can understand
(which is most of them) then you can transfer it to your Nook.
As for your local library, it depends on whether you can
access them online, or if you can do it locally.
There are, literally, tens of thousands of free eBooks available.
In the past month, I have siodeloaded close to 50 eBooks, all for
free. My big problem is finding the time to read them (or listen
to them in the case of the free audio books) before I download more!

I've also added several images to my Nook for wallpaper and/or
screensavers. Although they are B&W, the resolution is pretty good!