The cover browsing mode isn't there to enhance usability, it's there because it was fun to code. While my primary motivation for writing calibre is that I'm a voracious reader and I find it insanely useful, I wouldn't be doing it if I didn't have fun coding
Gentoo is actually pretty bad unless you use ~ARCH (even python 2.5 is ~x86). I will update the instructions for gentoo once the ebuilds hit at least ~x86