Cool Reader is an open source eBook reader, XML/CSS-based, which runs on Windows, Linux and Android devices.
Opening files can be a little awkward, because the program doesn't use native Windows dialogs, or accept drag and drops - you must grind your way through its built-in file browser, instead.
Locate your file and the situation improves. The book is automatically sized to fit the window; there's a nice animated page-turning effect as you move through it; a separate table of contents and bookmark system make navigation easy.
A "Night Mode" option displays light-coloured text on a black background, reducing glare.
A "Text to Speech" feature optionally reads your book out loud.
Cool Reader is exceptionally configurable, too: you're able to tweak fonts, colours, text formatting, page layout, controls and more.
The package also includes tools to help you browse online eBook catalogues, including Project Gutenberg and the Internet Archive, but these had some problems. We were able to view details of books, but not download or view them.
Cool Reader is an excellent and highly configurable eBook viewer. We had problems with some of its more advanced features, however, so you might have to spend some time experimenting to find out what works for you.