I know this is a really broad question, but I'm not sure where to begin. I'd like to learn a bit about Christianity from when it became the state religion of the Roman Empire until the Schism in 1054. Some of the more specific topics I was interested were the multiple councils, the different sects, the differences between the sects, what caused them to split, and how Catholicism and Eastern Orthodoxy began to evolve into the more defined different versions that we know today. Again I know this is very broad, so I'd love to hear about further reading on more complicated subjects.
Late Antiquity
Ecumenical Councils
General history
Early Creeds
Constantine:
These vary in how in depth they are. Brown and the Chadwicks are extremely readable and engaging. I have the resources to very easily give you titles on anything more specific you are looking for, so let me know if there is a person or topic in particular you are looking to pursue in more depth. Also, maybe you could further clarify what type/level of reading you are looking for.