If you're interested in morph based expression work, then i suggest getting a book called 'Stop staring' by Jason Osipa which is pretty much the bible of the method:

http://www.amazon.com/Stop-Staring-Facial-Modeling-Animation/dp/0782141293

Parts of it are quite technical (and you can ignore those if you want) but it also has a lot of discussion of how exactly we 'read' expressions and how to recreate the subtle signs in your own characters.