W.H.C. Frend's Rise of Christianity is long, academic, but excellent. Henry Chadwick's The Early Church is much shorter, but still excellent, and shows fine scholarship.

Texts written by conservative believers but of good scholarly quality would include Gonzalez's Story of Christianity: Volume 1, or Everett Ferguson's Church History, Volume 1, Ehrman's Lost Christianities presents the Bauer heterodoxy-first proposal in a more contemporary vein.

It's impossible, I would say, to find an unbiased presentation, that is the nature of history writing. But there are a number of well-researched texts to introduce this subject area. I would give some recommendations but most of mine are currently packed in moving boxes.