Do I have to buy eBooks on the nook through B&N?

No. The easiest way to get books is through the B&N store but you can also load other books on your nook. You can move files from your computer to your nook using a USB connection. The supported file types are ePub, PDB, MP3s, PDFs, and image files.

By embracing the ePub standard any non-DRM (copy protected) book can be loaded on the nook. Since B&N picked Adobe for their DRM support other places that sell - or rent - ePub books with that copy protection should also work on the nook. Sony recently converted over to ePub opening the possibility to use books from the Sony store on the nook.

Kobo eBooks & Overdrive library books have been tested on the nook.

To make buying books easier, and to get eBooks as a gift I'd suggest the new eBook B&N Giftcard!