Getting bus schedule information in Greece is a real hassle. Outside of Athens, I just try to get a general idea of how frequent buses between city 'a' and city 'b' are from guidebooks, and then go to the bus station as soon as I arrive at city 'a' and ask about schedules to city 'b'. Unsatisfactory, but I've never been able to figure out anything better.
In Athens you can usually get reliable information on buses in Attica and long distance buses between Athens and other major cities and tourist centers from the official tourist information office.
I've found that the Rough Guides usually have reliable information about how frequent buses are and how long the trip takes, but they don't give actual schedules.