Maybe I will try the modded version of Charis SIL, but it has to be modded big time in that case. I've downloaded the normal unmodded version, but I really don't like it. Some letters such as "k" look really weird. Just as many fonts do in Linux, the "<"-part of the letter is very, very thin. Other letters such as "o" and "b" and so on have problems with the round parts.
If I would replace Caecilia, then it would be by a version of DejaVu Serif with added weight. If I can find a freeware/open source font editor to easily do that.
Oh; and Trebuchet is part of any Windows installation. Don't know if it's on the Mac too, but it probably is: it's a core web font. Because of that, you can find it basically everywhere, allthough it's not officially free.
It is a font designed for reading on screen, just as Verdana is. (Verdana 10 has been my default Windows GUI font like forever now, but I find it too broad/round for long texts.)
The fact that Trebuchet is designed for low-DPI screens, is probably also the reason why it needs some more weight on the Kindle. The lines are thinner than expected, as the Kindle has higher dpi/ppi than the screens Trebuchet was designed for.