It's been a while since I had to convert Kindle books, but the last time I did - Calibre was the way to go.

It's a complete collection manager too so you can edit metadata, cover art, format and so much more.

It's definitely intimidating when starting out, but well worth it.

There's a looooong thread on mobileread forums about how to do this. Amazon changed their ebook format a year or two ago to make it harder to remove the DRM, but someone usually comes up with a new way to do it every time Amazon tries to foil them.

This is the thread I have bookmarked -- I haven't kept up with it all since I quit Kindle back when they removed the "download & transfer" option to let us save our own purchased property. But I think their newest format has been figured out now, so if it's possible to do, the instructions should be in that thread.

You need to remove the DRM from the books. I've heard Caliber can do this.

You can jailbreak the kindle and install OSS on it. For example KOReader. https://kindlemodding.org/jailbreaking/index.html

You can use dedrm plugin in calibre to remove the DRM on your ebooks. You just need to download them on your computer, like using an old version of kindle.

If you want to buy more recent ebooks, as someone else said, the DRM cannot be removed now, so you should download them from kobo if you want to still be able to download and remove DRM.

This is the most uptodate option i know of, using DeDrm/noDrm and Calibre, havent tested it myself though.

Supposedly you can’t use Calibre to remove DRM from Kindle E-books anymore since Amazon no longer lets you download the raw files to a computer. The only workaround I’m aware of requires you have a kindle device.

Of course you could find an alternative form of ebook acquisition

What open source reader are you using? I'd love an open source device to replace my Kobo, but the only option I ever found when I looked was basically to use an Android app, and that's not really what I hoped for.

if you want a reader to access your files, grimmory is the way to go, if you need to edit metadata before doing that or you want to import them to your ereader use calibre if you dont care about them looking nice you can just dump your files in the ereader tho), if you want to automate pirating them, there isn't a very good alternative to something like radarr, but ive been playing with bindery. these are selfhosted, there's also readest for desktop/mobile.