Unfortunately, the easiest way is to have an old kindle on your account with DRM that's been beaten. Then you can download the files like you're going to copy them via USB to that device, and use that device's serial with the de-DRM calibre extension and it's pretty easy.

I haven't found an equally effective way without it, though I haven't looked as much as the first method works for me.

You can rip Audible audiobooks with Libation. It's not applicable to ebooks, but worth just tacking on since we're on the subject of Amazon lock-in.