Our common categories of theology are: exegetical, Biblical, systematic, and historical. Exegetical deals with pulling meaning from the text. Biblical relates to the history of special revelation. Systematic is viewing that history as a single coherent unit we can study from various perspectives (think of a diamond). Historical relates to the history of doctrinal development.
Historical Theology ("HT") not truly theology. It's kind of a bridge between history and theology. It doesn't answer the question Who is Christ? it answers the question How did Christology develop in the 17th century in continental puritanism? It can be specific, so nearly all discussions are highly concentrated.
Another way to think about it is how Kevin Giles put it:
The study of the Scriptures does not explain why a doctrine emerged, how it was developed or what problem or question it sought to resolve. The history of the doctrine provides this information.1
One of the more pernicious myths out there relates to what we call primitivism. It's great when it shows up in Scifi where you have an ancient, yet incredibly advanced technology, but it's nonsense when it deals with theology. From Genesis to Revelation God lisped to His children, and from the students of the Apostles to even this today, we've been slowly developing theology. God didn't dump all revelation on Enoch and He didn't drop a systematic theology into the lap of Irenaeus. There were no church fathers, there were church infants. The church started out as a baby and is still growing while theology progresses along side. There is no profound, secret tomb under the Vatican. Not even the confessional period ended our development. Far from sealing the vault, our confessions actually act as a more stable foundation for further theological development. Historical Theology relates to the development from the theology of Irenaeus to the theology of the most recently published dissertation. The story is not over.
While systematic theology are historical theology are entirely distinct, the historical may serve a pedogogical model of the systematic. This why many systematic theology lectures are heavily reliant on historical data. Having a timeline to which we may attach data can serve to provide natural examples from the surrounding context. For instance, our core understanding of Christology was developed as answers to heresies in the 3rd and 4th centuries AD. The same could be said of Sacramentology in the Reformation. While we must understand the Reformed doctrine of the Supper as it stands in relation to other aspects of systematic theology, it may be easier to understand in contrast to Lutheranism.
The study of Historical Theology can be approached in myriad ways. One of the clearest is by what you might think of as a legal case. The History of the Christian Church by Philip Schaff mentions a great many of these cases, often associated with the defendant, which is usually the one classified as the heretic, but there are cases when history took a wrong like in the case of Lanfranc v. Berengar. Lanfranc was Anselm's predecessor and was instrumental in defending Transubstantiation. Some later important cases include Arminius v. Gomarus and Cocceius v. Voetius. Cases like these set the tone for an entire era.
Given its foundational, yet neglected nature of medieval history, I've separated works relating to medieval history and theology. You can view those in the Church History (Medieval) section.
Similarly, due to how incredibly specific the topic of HT relating to the post-Reformation (including Puritan) era can be, I've separated those resources as well. You can view that topic in the Historical Theology (Post-Reformation) section.
The Unaccommodated Calvin (Richard Muller)
There are a lot of books about Calvin, and for good reason. This one is core to Calvin studies. Richard Muller explains Calvin and his works in his own context. You don't know Calvin prior to reading this book.
The sequel to this is After Calvin by the same author. See the note about it in the Historical Theology (Post-Reformation) section
Iustitia Dei (Alister McGrath)
This is basically the history of the doctrine of justification. It's fairly exhaustive. At times he interacts with and attempts to correct The Harvest of Medieval Theology by Heiko Oberman mentioned in the Church History (Medieval) section.