Please introduce yourself here and give us some idea of what you hope to gain from this study.
"I keep hearing it come up in negative ways in my protestant church, but I want to explore it for myself before I form an opinion."
Mysticism is a vague word that some people use for some things that Christian mystics would also react to negatively. There is an element of Christian thought that can only be described as mysticism which is at the very heart of the Gospels, as when Jesus says that He'll dwell in us and we in Him. Some protestant denominations just don't think of that as mysticism. That's just a matter of terminology. Mysticism isn't found exclusively in the Roman Catholic and Eastern Orthodox denominations, though. I'm reading through the Theologia Germanica through this same CCEL. Someone told me that Martin Luther may have been the anonymous author, but I think it was written too early for that. He did have something to do with a publication of it. Come to think of it, if it was written before Luther, the author was catholic. Anyway, I pray you'll be blest for your Berean attitude of wanting to find out for yourself.
Relative to the negative reactions to mysticism, something can be learned from the story of Mary and Martha. (This isn't an original idea of mine, but I forgot where I read or heard it.) Mystics are like Mary. Sometimes we're at the feet of Jesus when others think we should be doing some work. Sometimes" and "work," are unfortunate choice of words, but to be brief, Mary doesn't respond to Martha, Jesus does. We have no defense, Jesus makes it right. Mystics are often misunderstood, as are also introverts.