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Amazon is making it impossible to remove the DRM from Kindle Books
(goodereader.com)
This is a most excellent place for technology news and articles.
"Almost all"... Unless you read a very specific niche, I've rarely looked for a book that I wanted to read and not found it elsewhere. There certainly are some that are specific to KDP, but hardly "almost all".
In fact just a few minutes ago I got another bundle from Humble that I loaded onto my kobo with no issue