Does anyone know of some good books or web sites about it?
I don’t think there is much to say about it. They used a method called Haruspicy, which consisted of reading their luck on the entrails of an animal. Usually they used a sheep or a dove. Then they would eat the flesh and burn the rest (they thought the smoke would feed the gods).
They also believed in Augury. A man would analyse the movements and songs of birds nearby, and from it he would try to predict the future.
In my opinion, Greek divination was much cooler. Specially the Oracle of Delfos, where women got stoned (they thought it was a trance induced by Appolos) and from their trip they would tell to people what was gonna happen.