This is really sad.
I've been using Marvin for many years and still do, every day.
Going to keep it for as long as I can.
As viable alternatives I can mention Yomu, Hyphen & Bookmate (you can upload your own books for free and use it just like Google books).
https://www.yomu-reader.com/
http://hyphenreader.com/
https://bookmate.com/