I recommend Stark's The Rise of Christianity. He was an agnostic when writing it, and it doesn't rely on any supernatural explanations for the spread of Christianity, just his own experiences with the growth of new religious movements.

Geza Vermes' Christian Beginnings: From Nazareth To Nicea may be worth a look; Vermes was a very prominent scholar of the Dead Sea Scrolls and the historical Jesus so he came to this project with a very strong background in Second Temple Judaism and the way it provided a context for early Christianity. Which is not to say he does not have his own very particular outlook on the development of Christianity(all scholars have their own particular outlook on what they study and attempt to defend a certain reading of the evidence) but his outlook is one that is well within the realm of academic discourse and he has a genuinely strong background in what he is trying to do.

For a good general history of Christianity, I would recommend Diarmaid MacCulloch's "Christianity: The First Three Thousand Years." A large portion of it is dedicated to early Christianity in both the east and the west. His book on the reformation is also good.

Honestly, The New Cambridge History of Christianity would probably be a good place to start. There are so many volumes, with numerous, well researched essays that are in step with modern (as of 2006 when the first volume was published) scholarly trends. Although the editors all have an agenda (who doesn't?), it's not going to be obnoxious.

If you have access to a university Library you should be able to access them online.

No one has mentioned 'Lost Christianities' by Bart Ehrman.