In this fascinating interview on Fresh Air, Ehrman notes that the Christians were not the first to call someone god. They began doing so at the same time Roman's began to deify their emperors.
And they tried to explain the Trinity as monotheistic by calling it modalism. Ehrman told NPR that "it's called modalism because it insisted that God existed in three modes — just as I myself at the same time am a son, and a brother and a father, but there's only one of me — well these theologians said: That's what God is like."
Erhman argues that the early belief in the resurrection of Jesus "is what initially led the followers of Jesus to believe that Jesus had been exalted to heaven and made to sit on the right hand of god." And, he says, "these beliefs were the first Christologies--the first understandings that Jesus was a divine being."
Ehrman also wrote several other books about Christianity including Misquoting Jesus and Jesus Interrupted.
Here's a review by Rob Bowman. And just below the NPR interview is Ehrman's interview with Interfaith Voices.