The latest generation, Kindle Paperwhite, Kindle Scribe 2, Kindle Scribe 1, and Kindle and the upcoming Kindle Colorsoft, all have something in common. When you plug them into USB into your PC or MAC, they no longer appear as external drives. This prevents users from using file managers to back up their books or to sideload new books onto the Kindle. Amazon has also removed the download and transfer via USB option for purchased ebooks from the content page. This will likely prevent people from stripping the DRM from the books and sharing them on piracy websites. This is not
If I were buying an e-reader these days I’d look at something like the Boox devices that run Android. You could install the Kindle and Kobo apps and a good third-party reader app (I like ReadEra), and have pretty convenient access to ebooks from any source. They are overpriced though.