This is the best general purpose reader app I've come across yet for Android.
I installed the F-Droid version which strips out all of the Internet-related stuff so it's just a reader that works with files on the device that's in front of you.
is a highly customizable and feature-rich application for reading books in PDF, EPUB, MOBI, DjVu, FB2, TXT, RTF, AZW, AZW3, HTML, CBZ, CBR formats on Android devices.