I am (or was) also in the same position - have to find a replacement for Marvin since it's gone from the App store. I know how I could transfer it to my next iPhone, but unfortunately there's no guarantee the in-app purchases will survive the procedure.
Have a look at
i2reader Cloud. Some more information can be found at
http://en.i2reader.net. I also posted some info about it in the Marvin part of this forum:
https://www.mobileread.com/forums/sh...&postcount=173
So far (2 days and counting



), so good. It does everything I need from an ebook reader.