A multitude of apps for reading ebook formats exists for the android platform. As different as they might be, they have one thing in common: they use the default font bundled with Android. The font selected by default is Roboto or Droid, which is not really bad, but not optimized for reading thousands of pages either. So, in my opinion, what all those reader apps really lack is a good font.
A quick search on the net revealed that some fonts exist which were designed with the specific purpose of reading ebooks in mind. Amazon has developed the Bookerly font which is delivered with their kindle devices. The Literata font comes with Google Play Books. Both are a noticeable typographic improvement over the fonts available as default on Android in terms of readability. The question is how to get these fonts into your favorite reader app?They can be downloaded as true type fonts. In order to use them, it is enough to copy the file to the /system/fonts folder. Adding new fonts is possible on android without installing any third party apps if you have a rooted device. Why are there special apps for things that are as simple as copying font files to the font directory? Wonders never cease.I managed to find the fonts on this site:
Which of the two fonts is actually better is very hard to say. Both are a great improvement over the standard Roboto font. I favor the Bookerly font, but ultimately this is a very subjective choice.