I've been trying out a couple ebook readers on linux. What stands out to me that so far only Calibre renders epub book contents exacty the way they were intended.
Both Bookworm and FBReader struggle with variable font formatting, especially syntax highlighted code in books from e.g. PragProg. I wonder if there's just an ebook reader with Calibre's rendering library?
Back in the days when my collection of books was very small I was looking for a simple reader that would just open files I click (e.g. the way Acrobat Reader does) rather than maintain a library. And what I have ended up using was Calibre e-book viewer. It actually is a separate program that comes with Calibre, you can associate relevant file types with it and use it to open them directly without using Calibre itself.
Give BookFusion a try and let me know if you have the same issues. BookFusion should also render epub content exactly as intended but also works on iOS, Android and Desktops(via Web). BookFusion not only supports epub2/3 but also MOBI, PDF, FB2, TXT and several other formats
