FBReader is a popular eBook reader that supports many eBook formats such as EPUB, Mobi, ETF, and fb2.
FBReader is not an open-source software, but it is avaiable for free for Windows, Linux, macOS, Android, and iOS.
FBReader uses its own book parser and rendering engine. The engine is lightweight, fast, and highly customisable.
FBReader is not open-source since 2015. For developers, we offer FBReader SDK: a library for building own ebook reader.