[kacir]

For quite a long time now I have been using Roboto. Roboto was created specifically for small screens of Android devices. It has a very neutral look and is very readable. I do not want a beautiful font for reading novels, I want a font that "disappears" when I read, so I can focus on the book.

If you insist on serif font, there is also Roboto Slab. It does have serifs, but it is slab-serif (like famous Caecilia on Kindles).

You might want to experiment with font weights - to see whether you prefer regular, medium or medium bold weight.

Roboto and numerous other fonts can be downloaded legally and for free at fonts.google.com
You can filter fonts you see, for example by number of styles (you want at least 4, so you have also bold, italics and bold italics), sort them by popularity, trending, time when they were added to fonts.google.com ... great site for discovering and getting new fonts.

EDIT:
have a look at Ubuntu font. I was using that one for quite a long time.