Is there a site similar to this for books not in the public domain? I wouldn't mind paying for ebooks if they were this nice and didn't lock me into using one application...
I think it would be hard for non public domain for one single reason - licensing and drm.
Similar to how Netflix/Prime video don’t let us download the movie file without their app.
There are non-public-domain ebooks out there without DRM. Kobo sells some, for example, though you have to check the format before purchasing since they aren't all DRM-free. Libreture has an extensive list of DRM-free ebook shops[1].