Google "Baymard Institute" and "buttons". They are usability guys.
Epic Bagel is a company selling a product but they are also usability guys. Look at their buttons.
If you can read Dutch I recommend reading Aartjan van Erkel. He also wrote a book (in Dutch) called Verleiden Op Internet (Seducing on the Internet) which discusses many things including calls to action.
Design-wise, I remember Zurb making some neat CSS3 buttons, but now everyone's got those.