Do you want the _exact_ book file you've downloaded or just _a_ file of the book you purchased (a license to read)?
For the latter, finding (main stream) ebooks isn't exactly hard, but it also isn't exactly legal.
For example I buy physical copies of books, which I don't have the space for. Then I donate them away and "acquire" the ebook version to read and archive. If an author sells the ebook directly without Amazon (or Rakuten) as a middleman, I purchase it from them directly.
Or should I just burn the book so I don't taint a potential book buyer? :D
In the extreme, someone could buy one DVD, rip it, and pass it on (for money or for free, it doesn’t matter) to a chain of hundreds of people who do the same and now all get to keep the movie digitally forever, while the copyright holder only sold the single initial copy.
My point is not to admonish anyone, just to point out how passing on the physical copy while keeping an uncompensated digital copy is inconsistent with the goal of compensating creators proportionally.
I couldn't make it work on books already downloaded to my kindle - they had to be downloaded via the app - and since it requires an old version of an app that automatically tries to update, I'm not sure how long it'll be possible, so my suggestion is to dump kindle and Amazon if you every want to have any control over your books.
[1] https://www.reddit.com/r/kobo/comments/1jbsf2n/comment/mhwhx...
I have to setup my Windows again and do it eventually though.