@sim0ne Yeah, it's quite a challenge to find DRM-free ebooks, but they are out there. I wish there was a one-stop shop, like Libro.fm for audiobooks. Some stores, like Kobo, offer DRM-free downloads when publishers allow. Here's one example, scroll down on that page and you will see Download options: EPUB 3 (DRM-Free). It might be worth visiting the author's website as well. Cory Doctorow, for example, offers DRM-free downloads.
When reading classic literature, remember to check sites like Standard Ebooks and Project Gutenberg. If the book is old enough to be public domain, you will almost certainly find DRM-free downloads there.
The original author of DeDRM stopped maintaining the plug-in a while back but there's a fork available in active development. It worked fine for me the last time I needed to remove DRM protection from a book, but that was back in March. So I don't know for sure that it would work today.