I'll certainly support Minion, which I never fail to admire, and the Caption variant works well on eInk. But it's not much use for those creating ebooks they want to distribute, as there's no way to licence it for embedding in an ePub.
There certainly is a variety of fonts that
do allow this, though, and fonts like Linux Libertine or Volkorn offer some very handsome alternatives.
A few other fonts worth knowing about are:
The Vera family - highly functional, but not really very elegant:
Bitstream Vera
DejaVu - an enhanced version of Vera
Arev - Vera which has been expanded to cover an extremely wide Unicode range
Legendum and Garogier - possibly a bit too light for use on an eInk screen.
Junicode - designed for use in Old English and also a bit too light.
The URW font set - A very wide set of fonts that includes some favourites like Garamond. Unfortunately this is
not licensed for commercial distribution, but as long as money isn't changing hands you can embed them in public domain ePubs.
A good list of free fonts can be found at
http://delubrum.org/.