Hey, after experimenting a lot with fonts, I've found one that makes reading very pleasant, and I haven't seen it mentioned much anywhere. The font is
Gentium, and it's free! I use it regularly in my lbook and in my gf's Bebook, and it's a quite nice serif font with a wide symbol support.
I've recently discovered a variation of this font called Gentium Book that is slightly bolder, and that seems specially appropriate for devices like eink readers (with less contrast than a screen). The problem is that this variation isn't (yet) as universal as regular Gentium, and it doesn't contain the full set of cyrillic symbols. That causes that it doesn't work with the Coolreader version that comes with most official firmwares (which doesn't like ttf fonts with no cyrillic support). I'm trying to quick-patch the font using a program like fontforge to "copy-paste" the missing symbols from regular Gentium, I'll post the result when I achieve something.
One final note, this font doesn't render so well using FBReader (OpenInkpot and some other firmwares), I don't know why, different rendering engines, I guess. It looks much better in CoolReader.
Well, just wanted to share my findings!