How Jesus Became God covers that process. Early Christianity was very complicated and divergent. Some groups thought Jesus was just a guy, others that he was just a guy who was raised to divinity, and still others that he was divine from the start. And then even among those who thought he had some sort of divinity, not all of them agreed with the trinity idea. And then Gnositcs come along and have a whole different cosmology about everything.

The Council of Nicaea didn't come up with anything on its own. It was an official stamp on what set of existing ideas were considered orthodox or not.