Defective by Design has an Article about getting DRM-Free eBooks which I recommend checking there.
Sometimes, Humble Bundle will have eBook bundles which couple I brought are DRM-Free. Just make sure you check the details of the bundles as not sure if every eBook bundles would be like that.
Can’t remember the exact number but I remember first discovered them on an ad break and they said it has Starships by Nicki Minaj. She is full of shit but that song feels nostalgic to me still.