Some of the fonts on google web fonts aren’t that great. And I’m not that good at picking the good ones from the not-so-good ones on first glance either.
Cardo is a really nice old-style serif font that I originally found recommended on some list of “the best of google fonts”.
It’s got a pretty good character repertoire for latin text (and I think Greek). The Google Fonts version doesn’t seem to include Hebrew, even though some other versions might? For library applications, the more characters the better, and it should have enough to deal stylishly with whatever letters and diacritics you throw at it in latin/germanic languages, and all the usual symbols (currency, punctuation; etc).
I’ve used it in a project that my eyeballs have spent a lot of time looking at (not quite done yet), and been increasingly pleased by it, it’s nice to look at and to read, especially on a ‘retina’ display. (I wouldn’t use it for headlines though)