The Early Church by Henry Chadwick is a great place to start. If there are particular aspects that interest you, it can be narrowed down, but Chadwick is a good general intro. Also worth considering are JND Kelly's Early Christian Creeds, Jaroslav Pelikan's The Emergence of the Catholic Tradition (100-600), Williston Walker's A History of the Christian Church, and Bengt Hägglund's History of Theology.