I believe that one of the best ways to respond to the idea that the Gospels are based on Greek myths and religion is to actually study the Ancient Greek culture. A great resource for that is Ancient Greece: A Political, Social and Cultural History.
This was a textbook from a classics course and so it provides the history of Ancient Greece at an undergraduate university level. It takes us right from the little we know of the oldest period, right unto the Hellenistic period and the first contacts with the Romans.
I found this to be a readable and accessible text that those without a university education will still benefit from. The descriptions are clear and it is filled with beautiful images.
While it does not address the Jesus Myth Theory, it is interesting to observe how it uses ancient sources. Classicists are not afraid to use texts written centuries after the events to piece together what really happened. They use a different standard for history than Mythicists.
