Outside of Reddit I’ve never heard anyone refer to Protestantism as if it were a single church. Nor have I ever heard someone introduce themselves as a Protestant. They always are more specific about the denomination.
For whatever reasons Redditors chronically get this wrong.
Protestants are a diverse group of Christians with no agreed upon leadership structure and no collectively agreed upon doctrines.
For that reason you can’t really get to know more about Protestantism because it’s not a single church.
On may learn about the history of Protestantism in general but you still eventually have to break it down into denominations.
As a good starting point I highly recommend the book Getting the Reformation Wrong: Correcting Some Misunderstandings by James R. Payton Jr.
Take a look at this book: Church of Rome at the Bar of History https://www.amazon.com/dp/0851517102/ref=cm_sw_r_cp_api_i_DCT728038NMV7P8HZDEX
And also this website: https://christiantruth.com/articles/articles-roman-catholicism/
Getting the Reformation Wrong: Correcting Some Misunderstandings James R. Payton Jr. places the Reformation in the context of medieval and Renaissance reform efforts, analyzes conflicts among the Reformers and corrects common misunderstandings of what the solas meant to the Reformers.
Church history:
1. The First Hundred Years AD 1-100: Failures and Successes of Christianity's Beginning is a good entry level, readable historical treatment of the Jesus Movement in First Century context to include the Jewish fight against despotic Roman rule and the violent separation of Christianity from Judaism.
2. History Of The Christian Church, a comprehensive, academic 8 volume history of the Christian Church. It’s not too academic and readable.
> I mean why he made a whole new denomination,
Luther loved the Catholic Church and didn’t want a new denomination. He was a reformer who wanted to reform the Church’s worldliness and corruption.
It makes no sense to claim he wanted to make a whole new denomination. You don’t risk your life trying to reform something you hate and want to leave.
You should really read Getting the Reformation Wrong.
> why not stick to catholicism, just without bad things that he meant...
Roman Catholicism refused to reform the bad things.
> If they expelled him, why would he accept that if they were flawed anyway?
Excommunication doesn’t care if you accept it.
If you are going for readability and doctrine, I would highly recommend J.I. Packer's Knowing God. If you are looking for an older, doctrinal resource, I would recommend The Westminster Shorter Catechism Explained from Scripture by the puritan pastor Thomas Vincent.
Do you have a source for this? In my reading of history I haven’t seen evidence of this. In fact the gospel of Thomas is a pretty recent discovery. There were many gnostic writing but there was never any contention that these writings were part of the Christian Scripture; in fact this was one of the key claims of the Gnostics, that they had a revelation special to them that was kept from the church as a whole. So there was never any debate to include the gospel of Thomas in the canon. I recommend a couple of resources to you: A New Eusebius is a source book of documents from the early church period (to 313), and includes readings from the Gospel of Thomas and other gnostic works
The Question of the Canon considers how the Scriptures progressed from a few books and letter received as words from God into their present “Bible” form. Lots of good research pulled together here.