I've been updating my fonts using the great hack from NiLuJe, but so far I haven't found another serif typeface that is as legible and readable at a small size on the Kindle as its original font (PMC Caecilia).
At first I tried Adobe Caslon and Garamond (classic print typefaces) but I find e-ink is definitely too low-resolution to make either of those seem crisp.
Currently I've switched back to Caecilia, with
Myriad as the sans-serif font. What are you using?