CoolReader is fast and small cross-platform XML/CSS based eBook reader for desktops and handheld devices. Supported formats: FB2, TXT, RTF, DOC, TCR, HTML, EPUB, CHM, PDB, MOBI. Platforms: Win32, Linux, Android. Ported on some eInk based devices.
Features

Read electronic books in FB2, TXT, RTF, DOC, TCR, HTML, EPUB, CHM, PDB, MOBI formats
Stylesheet support
A lot of customizations for better reading performance