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Mobi files are easily convertible to ePub without any loss (you can use for instance Calibre).

Buying Mobi files is sending a signal that DRM are OK.

I have a lot of DRM-free Mobi files. Do you just mean that .mobi are typically shipped with DRM? Because the same is true for ePUB too.

You are right. In my head, Mobi files were always downloaded from Amazon (and here, they always come with a DRM, right?).

The ePubs from the bundle are quite okay. I just tried one of the books with FBReader on Ubuntu.

Here is an example of an O'Reilly book in ePub format, as bought on Google Playstore: https://imgur.com/a/wPK7G. Clearly, nobody took a glimpse at this version before publishing. The second image is from an old JunOS book bought directly on O'Reilly website.

EDIT: Just bought the bundle, checked the Rust book and ePub is pretty good!

The ePub's look rather okay!