My CyBook is only a few days old, but I tested a lot of different fonts and these 2 are keepers for me and seem to be new for the crowd here:

Goudita Sans SF (http://www.fonts101.com/xt_search.asp?txtSearch=goudita+sans&Submit=Search&vt=Search)

Union (http://web.archive.org/web/20040225205030/http://us.uvm.dk/erhverv/specialpaedagogisk_st/)

BTW: The website where you can find the Goudita family is a very nice resource for free fonts.

I finally decided to use the Goudita Sans SF as my basic reading font... :)

Please see http://www.mobileread.com/forums/showthread.php?t=44269 for a general purpose thread on fonts.