Yet again a plug for Epubor Ultimate, which claims to, and in my experience, does remove DRM from pretty much any EPUB or PDF, and works on Kindle books as well. The workflow is much simpler than with the DeDRM plugin for Calibre, and so far it has survived at least one change in Kindle encryption. I have used it on books from Kobo, ebooks.com, and Amazon.It is pretty much the only non-free software I own, and it lets me have a completely free library.