I’ve done this successfully, but you need to jump thru quite a few hoops. These instructions are for Kindle only. I think dedrm can cope with Nook encryption but I don’t know how to do it. It was a while ago now, but to the best of my recollection:
Get yourself an older model Kindle. I used a Kindle 3 that I found at Goodwill for $20. You may have to rejig your home WiFi if your model doesn’t support the right version of WPA.
Register it to your Amazon account somehow. IIRC, you have to try and fail to do it from the device, then Amazon will email you instructions.
Install Calibre and setup the dedrm plugin. It’s pretty well documented.
Download what you want onto the Kindle. Pretty sure you have to use the Amazon website for this because of SSL issues on the older models.
Setup your Kindle with Calibre. This is also pretty easy and and well documented.
Grab the DRMed books off the Kindle into Calibre.
Run the conversion process in Calibre to remove DRM and convert to (for example) epub.
Optional: Jailbreak your Kindle and install KOReader to get a bit more value out of the obsolete brick you had to purchase to do this. This is a whole other set of hoops.
I’m not totally sure all this still works. You’re honestly probably better off getting a Boox device and downloading stuff from Anna’s Archive or similar.