It takes a bit of effort to track down and set up, but it works (and the only method I'm aware of that does).

I used this system last week to strip DRM from an iTunes book (which I then relocated into Calibre, cleaned up the metadata, then pushed it back to iTunes). Then you can copy the cleaned eBooks back to your original library/wherever. Requiem will automatically find your library/books and DeDRM them. Just sign into iTunes, authorize the 'computer', then download the book. Using a Windows VM image with an empty iTunes library will get you where you need to be. Requiem (from ismoothblog) only runs on Windows and you need a very specific version of iTunes installed to run it. If you want to read an iBook on your Kindle you will need both a DeDRM tool (Requiem) and a format converter as iBooks are Fairplay encoded ePub files.

Format conversion is optional as Kindle can read unencrypted Mobipocket books. DeDRM and archive the unencrypted version and you have no need to worry about the availability of a DRM compatible ereader or DRM removal tools until the next time you buy a DRM locked ebook.

Calibre remove drm off azw amazon books