Skyward from Brandon Sanderson. I finished the book in almost a day.. Which is insane compared to my usual reading rhythm.

And it isn't like epub is a closed format. Worst case you have to use a program to format your files. DRM is a much bigger concern for me. Luckily here in Germany, the big book sellers (with the exception of Amazon) have mostly given up on hard DRM for ebooks on their websites. Only invisible watermarks.