Finding a decent e-book reader software is not an easy endeavor, bit it looks like I found a reader which suits my needs pretty well. It’s called Calibre and, of course, it’s open source. I can’t say I liked the default layout and theme, but they ended up to be easily customizable. Book library UI is a bit outdated though, but it’s not a big deal for me since I spend most of my time reading stuff and not browsing the library.